Some new college grads setup interviews with 5 or so companies right around graduation plus previous internships that convert. That's the only time bar someone known in the industry who becomes available.
Why do you assume only new grads set up multiple interviews? In my opinion, it's never a good idea to accept a new job without interviewing at least a few other places to establish your market rate.
True, but when you are graduating, lots of companies are looking at you at you on their schedule, whereas later in your career, just the way schedules line up might make it hard to get more than two or three offers (if you are lucky) you could respond to at the same time...