Which part of "But now a righteousness from God is revealed which is by faith from first to last" do you not understand?
We evidently live in different worlds.
In the ideal world where you live, presumably righteousness comes from God.
In the real world where I live, it isn't that easy. There is a constant clamor from people declaring what is righteous, but these people disagree with one another. There is no evidence that I can find suggesting that God has left a clear guide to righteousness. However, amongst the clamor there are many who assert that they are expressing God's righteousness. But, again, those who assert that they express God's righteousness disagree with one another as to what that is.
If I ever happen to find myself in your ideal world, I will be guided by the principle you stated. But for now, I must find a way of guiding myself in the real world, surrounded by that clamor of contradictory claims.