The fool has said in his heart there is no God, not because he's come to an intellectual or rational conclusion that is neither intellectual or rational. The rest of the verse explains why. They are corrupt. They have committed abominable deeds. Here lies the real issue. Sinners don't want God because they don't want a just judge. They don't want accountability. They don't want an evaluation of their behavior. They don't want punishment. They don't want condemnation. So the fool says in his heart, not because it's intellectually sensible, but because he doesn't want to have to be accountable to any transcendent supernatural deity. There is no God, and that lets me be satisfied in my corruption and the committing of abominable deeds.