In my career, I have worked with several phD's who were very intelligent and went to very prestigious universities. But you would never know it. They never advertised it or bragged about it. They insist on you calling them by their first name. If you asked, they would tell you they have a PhD from xyz school, but they never go out of their way to let you know it

Then there are those PhD's that have contributed nothing to the advancement of the human knowledge of science that always, insist on being called "Dr" it does not matter how worthless their "research" was.

Most the the phD that are in the hard science and engineering, do it because that have a passion for the subject and are very good at their subject of expertise.

Most of the phD's in liberal arts / education etc, do it because they just want the title and unearned respect.