The reason is this:
When Jesus returns at the Second Coming (that's Revelation 19), he will return with the angels and his saints (that's the raptured church, those in white linen) and then he will judge the living nations. Those are the nations that remain after the Tribulation period of 7 years.
Not everybody will die. It says in Matt.24:22 and if Jesus hadn't come promptly - those days being 'shortened' - then there would have been no people left, but he will come soon for the sake of the elect (ie. the living believers) still living on the earth.
Then what will happen will be the 'sheep and goats' judgement - the separation of the people that remain (Matt.25:32) - when the goats (the children of the evil one) will go off to destruction and the sheep (Jesus' true followers) will enter his Kingdom.
Because, it's after that that Jesus will set up his Milennial Kingdom, which means a kingdom lasting a thousand years.
So, entering this Kingdom will be the Tribulation saints (both Jewish and Gentile), plus the glorified saints who came with Christ. Now, these Tribulation saints will be mortal, and in Isaiah especially, it talks about this period, and that they will marry and have children and live to very ripe old ages.
Now, the glorified saints have passed beyond the point where they would have any wish to choose to sin, but these Tribulation saints, and especially subsequent generations, are not above temptation. That is because each generation must choose Jesus as Lord and, whilst the parents might have done so, that does not necessarily go for the children.
That is why the evil one will be allowed to tempt them at the end of the thousand years. To prove forever what is in them. For this is the final test, to see whether they are all fit to receive immortal (glorified) bodies and go into eternity.
Because after Revelations ch.20 (to do with the Milennial Kingdom of Christ), comes ch.21, which is the coming of the eternal Kingdom of God the Father, and the holy city of New Jerusalem coming down from heaven to earth. No-one will enter that kingdom who is not perfect, in other words glorified.