Because there will be Gentiles who will receive Christ after the church has been gathered, as well as the nation Israel during the time of the antichrist and the time of God's wrath. The former are referred to as the great tribulation saints of whom the beast will be given authority to make war against and conquer. The church cannot be here during that time because Jesus took upon himself the wrath that every believer deserves and we are therefore not appointed to suffer God's wrath, ergo, we must be removed prior to the first seal being opened, which initiates God's wrath. Below is the information regarding the great tribulation saints:
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After this I looked, and there before me was a great multitude that no one could count, from every nation, tribe, people and language, standing before the throne and before the Lamb. They were wearing white robes and were holding palm branches in their hands.
Then one of the elders asked me, “These in white robes—who are they, and where did they come from?”
I answered, “Sir, you are the one who knows.”
And he said, “These are they who have come out of the great tribulation; they have washed their robes and made them white in the blood of the Lamb.
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Regarding this, if you will notice the word church is used throughout chapters 1 thru 3 and then never used again after the end of chapter 3. Coincidence? I think not. After the end of chapter 3 you will only see the word "hagios" translated as "saints" throughout the rest of the narrative. The meaning is that the church is no longer on the earth after the end of chapter 3, with chapter 4:1 being a prophetic allusion to the gathering of the church.
The spirit or type of antichrist has been hear from the beginning. But the literal man who will play the part of the antichrist is most certainly already in the world but has not yet been revealed. Regarding this 1 John 2:1-7 says:
"I say this because many deceivers, who do not acknowledge Jesus Christ as coming in the flesh, have gone out into the world. Any such person is the deceiver and the antichrist."
So according to the scripture above, anyone who is a deceiver and who do not acknowledge Jesus as coming in the flesh is a type of antichrist. But, there is a specific person coming who will be thee antichrist and who will also have a man who will be a false prophet who will be performing miracles on his behalf, as revealed in Rev.13:11-17
The Lord's promise in John 14:1-3 and 1 Thes.4:13-18 will take place prior to the beginning of God's wrath. The church can't be here for that, because God's wrath was already poured on Jesus, satisfying it completely.
Here something for you to consider regarding the gathering of the church prior God's wrath being poured out:
In Rev.17:14 we have the called, chosen and faithful followers returning with Christ to the earth to end the age, which would be a reference to believers within the church. If they are returning with him from heaven to the earth, then they would already have to be in heaven in order to return with him.
In Rev.19:6-8 the bride/church is seen in heaven at the wedding of the Lamb receiving her fine linen, white and clean. Then in Rev.19:14 we see the bride/church following Christ out of heaven riding on white horses and wearing her fine linen that she will have previously received.
If the church/bride is receiving her fine linen at the wedding of the Lamb in heaven and the church is seen following Christ out of heaven, then she would already have to be in heaven.
