Plainly put,the Bible says that the saints will not be gathered unto Christ until there is a falling away first,and the man of sin steps in to the temple of God,and claims to be God,and then rules over the beast kingdom,and makes war against the saints,which the Bible says the saints shall be given in to his hands for three and one half years.
The reason for this is,as long as the world can repent of their sins,God will not take His children,and for the first half of the 7 years tribulation they can repent of their sins,but when they take the mark then salvation is no longer available to them,and God can take His children,but God also allows the beast kingdom to make war against the saints,which the Bible says when they shall scatter the power of the holy people,then all the things are finished,and the man of sin shall destroy the mighty,and holy people(Daniel 8:23-27).
The saints being on the earth is the truth on the earth,for if the saints are gone pre-trib,then the truth is gone during the tribulation,but the Bible says the man of sin casts the truth to the ground(Daniel 8:12),but if the saints are gone there is no truth to cast down,and when he addresses the world,that he has the answer to solve the world's problems,and provide peace,he blasphemes God,and His tabernacle,but if the saints are gone there would be no need to do that,because the truth will be gone,which he is telling the world do not follow that pesky Christian religion,for there is no personal God to help you,and no prosperity there,in an attempt to get as many people as possible to take the mark.
Some people use the reasoning that God is love,and would not allow the saints to go through the tribulation,but that logic is faulty,because if they say that,then God would of not allowed any saint to be persecuted all through the history of the Church,because persecution is persecution.It is not going to more hurtful in the tribulation,and not more suffering on an individual basis,but when Jesus said that there would be persecution like there never has been on earth,it does not mean it will be worse for an individual saint,than what saints have went through in the past,but it will be on a bigger level,for it will be the saints worldwide being persecuted,and not in certain areas like now.
Like Jesus said when He resurrects to heaven the saints will do greater miracles than Him,but how do you out do Jesus on an individual level,but it means when Jesus resurrects to heaven,it will be on a bigger level,for the saints will impact the world,where Jesus was only sent to the lost sheep of Israel.This greater persecution is not more hurtful,but on a greater level.
Besides it is called persecution during the tribulation,which has to refer to the saints,not the world,because the world is following the beast kingdom.The world's not being persecuted for they are together going against the saints.The tribulation is concerning the saints being persecuted,not tribulation for the world.