Why are we not sinless until our physical body dies?

Christianity has always opposed any religion that considered the soul purer or holier by nature than the body, as some eastern religions or Gnosticism did. At the same time Christianity through new birth introduced two natures in a man, at war with each other calling, one the flesh and the other the Spirit. (Gal 5:17) Most biblical commentators I have read make sure it is understood that the term ‘flesh’ in scripture is not just referring to the physical body but by the word ‘flesh’ it means the whole evil nature, soul and body, to make sure the idea of a false Gnostic conflict between soul and body is avoided. To muddy the waters slightly, there is a general admission through the Lords prayer that Christians still need to pray daily: give us this day our daily bread and forgive us our sins as we forgive those who sin against us. There may be some that think they might not sin a whole day but most Christians, at least the ones I have met, would agree with the concept that they are still very much guilty of daily sins against God’s perfect law after conversion. Only that by the blood of Christ are they cleansed and forgiven by their confession. But even those that believe in some kind of Christian perfectionism and imagine to be sinless for several days or longer would still understand that they frequently sin, just as James says in James 3:2. I personally feel I perpetually fall short of obeying the Law fully as I doubt I have ever loved God more than half as much as I ought to. But that is just my own persuasion along with several famous Protestants mind you. On the other hand, there is a general belief that after we die physically, that we will never sin again. In summary, common Christian beliefs. While in the body: 1 John 1:8-9 (ESV) > If we say we have no sin, we deceive ourselves, and the truth is > not in us. If we confess our sins, he is faithful and just to > forgive us our sins and to cleanse us from all unrighteousness. After we die, in heaven Revelation 21:4 > He will wipe away every tear from their eyes, and death shall be > no more, neither shall there be mourning, nor crying, nor pain > anymore, for the former things have passed away.” So the question is: if the word ‘flesh’ is not highly linked to the physical body, why is this the case (without promoting any kind eastern religious or gnostic ideas) that, while we live in the physical body, we can’t avoid sin and after our soul leaves the body into heaven (or rather with a new body) we are sinless forever? Why does this shift in imperfect-to-perfect, occur, exactly when the physical body dies?