Why do people who believe in God live their lives as if there were no God?

Some corrupt churches teach us to remain in our sins.

Well, aren't you full of yourself.  Many believers DO lead their lives according to God's directions.

1 John 4:1 

Beloved, believe not every spirit, but try the spirits whether they are of God: because many false prophets are gone out into the world.

This is my honest perspective:

I know that God lives, not because I have seen Him but because of powerful revelation from the Holy Ghost. Revelation from the Holy Ghost is actually more convincing and has more staying power than seeing. Doubt can follow sight.

I try each day to pick myself up and do better but I continually fail and commit sins that I know that I would not commit if Jesus was coming tomorrow. So yes, I am a procrastinator. It is characteristic of this fallen world. Fortunatley, I know that I am sincere about repenting and I know that God knows that no one is going to be perfect in this life and that He is merciful towards those who are trying. There is hope becasue of the all reaching power of the atonement. But if I stop trying and no longer care about what God thinks, then hope and trust that God will save, disappears.

People who believe in God often enjoy committing sin and are unwilling to give up sin.  Maybe they actually do think they'll repent late in life and avoid eternal damnation.  Further, some Protestants erroneously believe faith is sufficient for salvation--that one doesn't have to give up sin or make any attempt to live a morally virtuous life.  As Jesus told us, the path to damnation is wide and numerous people find it.  The path to salvation is narrow and few find it.  Padre Pio told us, however, that one who prays much is certain of avoiding damnation; one who prays little puts his salvation in jeopardy; and one who does not pray is certain to be damned.

"Not perfect, just forgiven" is their mantra.

Because there is no god.