Romans 1:20, 28

There are things about him that people cannot see—his eternal power and all the things that make him God. But since the beginning of the world those things have been easy to understand by what God has made. So people have no excuse for the bad things they do.

People did not think it was important to have a true knowledge of God. So God left them and allowed them to have their own worthless thinking and to do things they should not do.