The short answer to this question is that they lack spiritual understanding.
But, beyond that, there are a few reasons. First, they listen to false teachers/pastors who also have no spiritual understanding (christianity, being a counterfeit of the true faith, is full of them).
Next, they don’t realize that there are three spiritual harvests, which are tied to the three times a year the Israelites were required to go to the temple in Jerusalem (the people of Yah assembling together in the house of Yah), so they confuse passages of scripture that speak about one harvest, and conclude that it is the only harvest.
One example of this is Matthew 24, which speaks about things that will occur during the tribulation. Those who lack spiritual understanding don’t see that, in this passage, Messiah is not speaking about the bride (barley), but about a different group of people—the tribulation saints (wheat).
More specifically, those to whom He is speaking there are physical Jews, not Gentiles. The bride is made up mostly of Gentiles, but, within the tribulation saints, there will be a very specific group of physical Jews who will be sealed by the Spirit in the same way the bride was.
They will be virgin men who are physical Jews, and there will be 12,000 of them from each of the 12 physical tribes—144,000 total. They will be the only people during the tribulation to be sealed by the Holy Spirit. The rest who believe will have to “endure to the end” to be saved (they will be saved at their deaths).
And, finally, they don’t really know what a “rapture” is—they think it applies only to a single group of people. Some even claim that, because the English word “rapture” doesn’t appear in their English translations of scripture, it must be false. That comes from the shallowest of intellectual activity. The scriptures were not inspired in English, so what must be understood is what the word itself means, and then the meaning of that word discerned in what the scriptures state.
The word “rapture” means “to be caught up, or carried away.” That is exactly what happens each and every time a believer physically dies—his soul is caught up, or “raptured” to heaven. So, the rapture of the bride has been happening since Acts 2, when the Holy Spirit was poured out as the New Covenant.
What erroneously gets labeled the “rapture of the bride” is, specifically, the final day of the barley harvest, when the barley (bride) that remains in the field is all gathered together at once. But, every believer who has died has already been raptured.
So, the bride has been raptured. The bride is being raptured. The bride will be raptured. All three statements are true.
“Come up hither” ends the barley harvest in Rev. 4:1, and it ends the wheat harvest in Rev. 11:12.
For more information: Will There Be a Pre-Tribulation Rapture?