People start to believe in God when they hit the bottom. Or when they feel like that nothing they do can help them from the problem in front of them.
So, until and unless one is cornered or hits bottom, they wont realize the importance of god. And smart people are people who go through life, with smartness and using brains. So most of the times, they dont believe in god.
But I dont actually agree with you.
These great people have believed in God.
Now if you believe there are other people who are smarter than them and still choose to not believe in God, I guess, it is what it is.