How common are PhD interviews in the U.S.?
In my experience, it's typical for a US university to invite PhD applicants for a visit only after they have been accepted for admission. The purpose of this visit is primarily for the school to make a good impression on the student and try to convince the student to accept their offer of admission. An interview is not normally a required part of the application process.

However, one of the best things you can do to increase your chances of admission to a particular graduate school is having a professor there who wants you to work with them, and who will lobby the admissions committee on your behalf. In order to get this kind of connection with a professor, you will need to meet them and establish a rapport well in advance of applying to the university, and it's possible that the professor will want to meet you in person as part of that process.