Among the phd professors I know, none of them go by "Dr."

What I've found is that it's usually those of lesser degrees who insist on being called "Dr.", for whatever reason.

I think it's because the ones who actually know their stuff don't feel the need to rely on credentials. They're comfortable that they can establish everything they believe from the arguments. Thus, touting your credentials is a sign that you don't trust in your ability to establish your own aguments.