Concerning your question/comment as to why are these Christians still here on earth during the Tribulation really does have a simple answer. But not everybody wants to believe the why.
In Matthew 23, we see several people (the virgins) were expecting the messiah (groom) to come and were watching and even waiting. (In other words, they are expecting the rapture.) But, and here is the answer in a nutshell, some were foolish and enjoyed the cares of the world while waiting. They were not sold out to Jesus and it cost them their close relationship with the groom. You see, it's like a family (physical or spiritual). All of the family members are family but only certain ones can claim marriage rights to the groom. The one reserving themselves just for the groom. Many Christians will be left behind due to the requirement to be Holy as He is Holy.
Those left behind will either fight to the end or give up and be lost forever. I'm truly afraid many in the church will be left after the rapture, but prior to it's happening, they had believed all was well with their soul. Reminds me of a song many years back: "I wish we'd all been ready". Now I understand we must hunger and thirst after His righteousness. I write this not as judge but as one wanting to have a pure heart ready when He appears.
Hebrews 12:14: Make every effort to live in peace with everyone and to be holy, without holiness no one will see the Lord.
Jesus gave a parable of the Ten Virgins in Matthew 23:1-3. At that time the Kingdom of heaven will be like ten virgins who took their lamps and went out to meet the bridegroom. Five of them were foolish and five were wise. The foolish ones took their lamps but did not take any oil with them...