Why do some people believe..... that you can't be a scientist and still believe in God ?

Quite frankly, there's a marketplace of gods. Einstein's views aligned more with Spinoza's God than anything promoted by religion, which definitely feeds confusion into the system. The overwhelming consensus on what God even is boils down to some anthropomorphized overseer, which many prominent scientists have spoken out against. Even Einstein thought that religion was basically a collection of fairy tales.

But to go back to the first point, God is a fairly personal topic which people interpret in a wide variety of ways. Science is considerably less subjective and focuses its efforts in finding objectively consistent answers. It's two very distinct patterns of thought applied to each topic.

People are pretty good at noticing inconsistencies. When we observe that one line of thinking is applied to one thing and a completely separate line is applied to another, it's almost intuitive to believe that both things are incompatible.

The truth is usually messy, and it's in this messy truth that we can understand that people can think about different things in different ways. You can be flexible enough to apply different standards of scrutiny as you see fit. It's not a zero-sum game that requires going all-or-nothing