Smart people may have been, and may be religious, but I don't agree that _many_ smart people are religious, at least not dogmatically religious. In my experience _many_ smart people are not religious. As is the OPs experience. You can always find outliers.

And I wouldn't call Einstein religious, at least not the late Einstein. Nor would I call having "humility with respect to the question of human existence" being religious.

Basic inquiry is a trait of smart people. Dogmatic religion requires, maybe even demands for its own existence, that most enquiry (i.e. the "why" questions) be put aside on faith alone. Smart people just tend to not accept the "faith" argument. Smart people tend toward experimentation and logic, over blind faith.