Google's policy of separating the interview process from the job assignment breaks a lot of the incentives to interview that most companies have in place.
Most managers, if given a choice between hiring a "good enough" candidate and hiring no one, will choose to hire. Particularly in a growing company, where everyone is overworked and there's a lot of grunt work that people would love to have a new recruit do. Over time this results in a degradation of overall employee quality ("A players hire As, Bs hire Cs") and a bunch of deadweight that were needed to solve a temporary staffing crunch but later end up holding the group back.
It also tends toward fiefdoms and empire-building; when you're hired by the department, it can be very difficult to move between departments. One of Google's strengths is that transfers are pretty easy; I doubt that would be true if you were brought in by a department manager instead of assigned to the company as a whole.