Indeed the Trumpets are NOT just some isolated scattered moments. They give warnings to all, with messages to all generations.

Especially that first trumpet!

It is absolutely key to the WHOLE salvation story, God's covenant to save all He possibly can save, and to realize this earth is on fast track to destruction.

Consider the historicist interpretation dealing with Jerusalem:
Revelation 8:7 The first angel sounded, and there followed hail and fire mingled with blood, and they were cast upon the earth: and the third part of trees was burnt up, and all green grass was burnt up.

Green grass
Grass can be a symbol of the fragility of human life, but it says it's GREEN Grass -- Green grass is also symbolic of rest and peace from God, as implied in Psalm 23:2, where the Bible implies that God will "make us lie down in green pastures". Green pastures of rest and peace.
Jerusalem -- the very name means peace and rest.
It was God's higher plan for Jerusalem to be the center of peace, a place where people could find the rest and peace only God can give . But when they rejected the "Prince of Peace" all their peace was burned up.

Luke 19:41 And when Jesus drew near to Jerusalem he beheld the city, and wept over it,
19:42 Saying, If you would have known, even you, at least in this your day, the things which would have brought you peace! but now they are hid from thine eyes.
19:43 For the days shall come upon you, when your enemies shall cast a trench about you, and compass thee round, and keep thee in on every side,
19:44 And shall lay thee even with the ground, and thy children within thee; and they shall not leave in thee one stone upon another; because you knew not the time of thy visitation.

To me this historical event is a warning --
A third of the trees (the leaders) were burnt up
The green grass was ALL burnt up

Their peace was gone. God's chosen people lost their peace and their city of peace, because they rejected the "Prince of Peace".

The message is: come to Jesus, accept Him as Lord and Savior.
Look to Him for He is life, He is everything we need to have life, hope and peace, not just now, but for all eternity.
Surrender yourself to Him, accept Him, follow Him, trust Him, make Him Lord of your life!
The world as we know it, is heading for destruction, and no one can escape that destruction EXCEPT through Jesus who came for just that purpose -- to seek and save sinners. If only they will accept Him, accept Him as LORD and Savior and follow Him.

Jerusalem (the old Jerusalem) is not the city of peace, it's peace is gone, burnt up, and any supernatural being manifesting itself there is NOT the real Jesus.
Why is there no peace there -- why will there never be any peace in old Jerusalem?
There is no peace without Jesus.

I don't understand why people reject or don't see this First Trumpet Warning for it's the very foundational warning concerning judgment.

Another writer puts it better than I can:

The fall of Jerusalem is the most profound example of both Christ's great love (read the gospel of John and hear the very words of Jesus calling the people to come to Him for LIFE and salvation). The final years of witnessing, and the withdrawal of the Holy Spirit, rejected, ceasing to plead with those who reject Christ. Before the armies even came, people, no longer guided by the Holy Spirit were doing terrible things. Yes, Satan, let loose is a terrible, terrible thing.

It was horrendous, even before the armies ended it. But ... all who were following Christ in trust and obedience were saved from that destruction.

That first trumpet is not some isolated scattered event. It is a loud and a full depiction of the tremendous danger of following Satan's lures, and neglecting the only protection, the only source of LIFE available to mankind through Jesus Christ, the ONE by which we must be saved, there is salvation in no other. (Acts 4:12) Yes, the 1st trumpet was (limited in the area affected) in the end the whole earth will be involved. All the trumpets were limited in their area ,(though they process and enlarge in the area they cover with each trumpet) which actually proves their historicist role --

The seventh trumpet also begins at a certain point in earth's history, 1844 when judgment begins in the courts of heaven. And it (the seventh trumpet), being the last, covers ALL the judgment scenes in which books are involved. Beginning with the heavenly court scene," ( the time of the dead, that they should be judged) it takes the reader to the second coming, when Christ delivers His people and the earth is left in ruins, {with thunder and lightning and voices) and on to the third coming, a thousand years later, when ALL will appear before the white throne, followed by all traces of sin burned up. The mystery of salvation is completed, The kingdoms of this world become the kingdom of our Lord, and of his Christ; and he shall reign for ever and ever. Then Christ is KING here on the earth which will be completely made new. Peace will be restored forever. That's when the mystery of salvation will be completely finished, the redeemed inherit the earth, and sin will never be again.

The trumpets are not just to let us know there is a time of intense trouble ahead. The real purpose of those trumpets is to warn people that they desperately need a Savior, they need to turn to Jesus.

He is the prince of Peace -- His New Jerusalem is the real city of peace. Like Abraham we follow Christ not looking for a literal earthly city but for, " a city which hath foundations, whose builder and maker is God." Heb 11:10

We can find many more, but suffice it to say that Protestant Reformers Martin Luther, John Calvin, John Knox, John Wesley and other reformers were ordained by God to do a great work but, unfortunately, the church which arose after them, having restored lost Bible truths, was not willing to search for other lost truths or to include those found by others into their creeds. They locked down into their own creeds and refused to look any further. John Robinson summed it up in these words, as he charged the Pilgrim Fathers:

"If God should reveal anything to you by any other instrument of His, be as ready to receive it as ever you were to receive any truth of my ministry; for I am very confident the Lord hath more truth and light yet, to break forth out of His holy Word. For my part, I cannot sufficiently bewail the condition of the reformed churches, who are come to a period in religion, and will go at present no further than the instrument of their reformation. The Lutherans cannot be drawn to go beyond what Luther says; . . . and the Calvinists, you see, stick fast where they were left by that great man of God, who yet saw not all things. This is a misery much to be lamented; for though they were burning and shining lights in their time, yet they penetrated not into the whole counsel of God, but were they now living, would be as willing to embrace further light as that which they first received.... take heed, I beseech you, what you receive for truth, and compare it and weigh it with other scriptures of truth before you accept it; for it is not possible the Christian would come so lately out of such thick antichristian darkness, and that full perfection of knowledge should break forth at once.[vii]"https://oll.libertyfund.org/titles/robinson-words-of-john-robinson-robinsons-farewell-address-to-the-pilgrims

It seems the issue here is almost the same as it was 2000 years ago.

People want Christ to set up His kingdom and start ruling the present world and straighten out the people and nations to live in subjection to that rule.

They've neatly tucked the verses we understand happening at the second coming -- those about meeting Christ in the air (etc) and Jesus taking His people to His Father's house, as a short duration of a "rapture" taking place a short time (seven years) before the second coming, so the real Christians don't have to be here during the tribulation, but they will come back with Christ at the second coming and Christ and His armies will fight against the antichrist and the false prophet and their armies, and He will set up His kingdom, resolving all the issues in the "tribulation" etc.


Why is this a problem, since both (Adventists and Rapturist) groups believe Christ is coming again?

Could it be because it opens wide the door for Satan to personate Christ, and deceive the world into worshipping him?