Do universities prefer not to hire their own PhD graduates in faculty positions?
At my university - or at least in my department - the norm seems to be that they prefer to hire externally for tenure-track jobs (speaking from experience serving on a search committee for a tenure-track position). The line of reasoning is, "if we're the ones who taught/mentored/molded Candidate X, then they're not bringing in anything new in terms of scholarship."

This is less applicable to non-tenure track jobs, as those faculty members are usually teaching lower level classes and don't have any research obligations. There are several PhD faculty members in my department that were hired internally who work as lecturers.

Anecdotal, of course, but I've studied at/worked at a few universities over the years, and this is the pattern I've tended to see. Tenure-track: external. Non-tenure-track: internal OK.