Yeah, this could be a function of not just the intellectual curiosity of the student (self-selection, really), but of the effectiveness of the career services of the school – getting them great jobs coming out of undergrad.
The fly in the ointment of the “great career services” argument is a school like Princeton – of course they have a great career center, yet they still have a high proportion of PhD choosers. So it isn’t always or simply that. For at least some PhDs – maybe most (unknowable) – the motive is intellectual curiosity, appetite, and stamina.