It's done that way to align incentives. The hiring manager's incentive is always to fill a role; if they didn't have more work than they can do, they wouldn't have open headcount. This biases them toward hiring fast and lowering the bar - not so much that they can't work with the person, but enough to overlook minor red flags in the interest of filling the position. In other words, the inefficiency is a feature, not a bug: the company doesn't
want to hire fast if it means sacrificing quality.
It also has the added benefit of improving internal cooperation, trust, and mobility. Many big companies suffer from the attitude of "Oh, those doofuses in department X can't be trusted to get anything done right". [1] When you have a common hiring process, you know that everybody in the company passed the same bar you and your teammates did, and your baseline assumption should be that they're as competent. And similarly, transfers get pretty easy because everybody has already passed a common hiring bar, so you know that the candidate is already qualified technically, you just need to judge if they're a good fit for your team.
Time to find a mutual fit can vary a lot. It was really quick (like a couple days) my first time around, because I was early enough in my career to be pretty adaptable, had a very in-demand skillset (Javascript), and had multiple teams - Search, GMail, Docs - all vying for me. It was a lot slower (month+) my second time around, because I interviewed in April 2020 as the world was falling apart from COVID and teams kept disappearing as they got canceled or their headcount got cut.
[1] https://i.insider.com/50a25c45eab8ea8c41000000?width=589&for...