Most companies hire by department, so a good manager is incentivized to do a good phone screen (or at least make sure a good phone screen is done), because it is their people who waste time on a plant interview of an unqualified candidate.
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.
That's intentional, because it also breaks many of the misaligned incentives of departmental hiring that result in pathological organizational behavior.
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.
From the corporate side, not having managers do their own hiring seems to make sense. But from the employee's side, it means that you don't get a chance to meet and evaluate the manager and team who you'll end up working with. I'm sure that Google is similar to most companies in that the quality of managers varies widely and different departments have very different work and environments. Not being able to evaluate the manager and team that I'd be working with if I were to accept a job at some company would be a deal-breaker for me.
For me it wouldn't necessarily be a deal-breaker, but I would demand a significant premium on salary or cash signing bonus for the risk of not knowing the group and manager I'm going to be working with, or the work that I'm going to be doing. That seems like a pretty bad incentive for the hiring company too.
There are some pretty simple solutions to the problems that you mention. For example, Amazon's bar raiser program was introduced specifically as a check to the power wielded by managers and to ensure a consistent hiring bar across the entire company. A bar raiser has veto power over the candidate which cannot be overruled by anyone.
It also breaks incentives in the other direction - as a candidate you're interviewing for "a job" but not "a particular job", so you're taking it on faith that it'll align with your interests. If you're considering other offers as well, you're weighing a known quantity against a pure ethos play.
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.