Two reasons why the rapture has to be pre-tribulation:
1. The rapture is imminent, i.e. Jesus tells us that we don't know when he will come, it could happen at any moment - there are no events or signs that must precede the rapture. Therefore the foretold events of the Great Tribulation cannot start before the rapture has taken place because that would be a sign that the rapture was going to occur in the very near future.
Matthew 25:13 (WEB):
(13) Watch therefore, for you don’t know the day nor the hour in which the Son of Man is coming.
Matthew 24:44 (WEB):
(44) Therefore also be ready, for in an hour that you don’t expect, the Son of Man will come.
2. The "restrainer" has to be removed before the Great Tribulation can occur. See 2 Thessalonians 2 (below). Paul is correcting Christians who had thought that Jesus had already returned and was present, and that the events of the Great Tribulation were about to begin, so he tells them that before Jesus returns for the Church there will be a falling away from the truth (apostasy), and that before the Great Tribulation can begin the antichrist must be revealed. The apostasy occurred later (a long time ago now!), so the rapture can occur at any moment. The antichrist has not yet been revealed (that would be a sign or event that would make the rapture no longer imminent).
2 Thessalonians 2 (UASV+):
(1) Now we request you, brothers, with regard to the coming of our Lord Jesus Christ and our gathering together to him,
(2) that you not be quickly shaken from your composure or be disturbed either by a spirit or a word or a letter as if from us, to the effect that the day of the Lord has come.
(3) Let no one deceive you in any way, for it will not come unless the apostasy comes first, and the man of lawlessness is revealed, the son of destruction,
(4) who opposes and exalts himself against every so-called god or object of worship, so that he takes his seat in the temple of God, showing himself as being God.
(5) Do you not remember that while I was still with you, I was telling you these things?
(6) And now you know the thing restraining him, so that in his time he will be revealed.
(7) For the mystery of lawlessness is already at work; but only until the one who is right now acting as a restraint is out of the way.
(8) Then the lawless one will be revealed, whom the Lord Jesus will do away with by the spirit of his mouth, and wipe out by the appearance of his coming,
(9) namely, the one whose coming is in accordance with the activity of Satan, with all power and signs and false wonders,
(10) and with every unrighteous deception for those who are perishing, because they did not receive the love of the truth so as to be saved.
(11) For this reason God is sending upon them a powerful delusion so that they will believe the lie,
(12) in order that they all may be judged because they did not believe the truth but took pleasure in unrighteousness.
(13) But we ought always to give thanks to God for you, brothers beloved by the Lord, because God chose you as the firstfruits for salvation, through sanctification by the Spirit and faith in the truth.
(14) To this he called you through our gospel, to the obtaining of the glory of our Lord Jesus Christ.
(15) So then, brothers, stand firm and hold fast to the traditions which you were taught, whether by spoken word or by letter from us.
(16) Now may our Lord Jesus Christ himself, and God our Father, who loved us and gave us eternal comfort and good hope through grace,
(17) comfort your hearts and establish them in every good work and word.
The Great Tribulation starts when the antichrist makes a peace treaty with unbeliving Israel - the start of the last 7 years of the appointed 70 years for Israel (Daniel 9:24) - Daniel 9:27 (WEB):
(27) He will make a firm covenant with many for one week. In the middle of the week he will cause the sacrifice and the offering to cease. On the wing of abominations will come one who makes desolate; and even to the full end, and that determined, wrath will be poured out on the desolate.”
So the antichrist must be revealed before the Great tribulation can start. Today, the unbelieving Israel have returned to their land, but the "restrainer" is still preventing the antichrist from appearing. The "restrainer" is the Holy Spirit. The Holy Spirit permanently indwells Christians:
John 14:16 (WEB):
(16) I will pray to the Father, and he will give you another Counselor, that he may be with you forever:
Romans 8:9 (WEB):
(9) But you are not in the flesh but in the Spirit, if it is so that the Spirit of God dwells in you. But if any man doesn’t have the Spirit of Christ, he is not his.
Therefore, for the Holy Spirit to be taken out of the way, all Christians, and the restraining ministry of the Church, must be removed from the earth - the rapture. So the rapture must occur before the Great Tribulation.