My solution is smaller companies - that way the underperforming ones can be eliminated more easily.
To ensure this, someone experienced with interviewing from outside the team/organization, so called bar raiser, is always added to the interviewing team. This person is there to ensure high quality hires and has hiring veto power.
The rest of the interviewers are from within the team, likely under pressure to compromise on quality when they're desperate enough to hire someone, but they can't do it unless the bar raiser agreed.
That's just one data point, and maybe what you're describing is for more senior hires?
At this point it's Kool-aid. In practice bar raisers have so much more interview experience that you can very easily pick them out, but so much of their behavior results in simply weeding out the bad.
I have received LOTS of offers from Amazon over more than a decade (only one accepted at one point), and my friend used to be a bar raiser. In his first interview at a new company he was both rude to an interviewee and flat-out wrong technically on multiple points along with it.
You can't raise the bar if you can't pick the bar raisers.
If this is raising the bar, no fucking thank you.
To be honest, if a large company has multiple departments that essentially govern themselves, perhaps they could just give them proper names, as if they were subsidiary companies? Because it seems to me that the mechanism behind the "best performers leave when they feel the average performance of workers in the company goes down" is really bounded by the company name. I.e. people in department of frobnification leave because of the people in department of foobing bars. But people in Frobnificator perhaps wouldn't leave because of people in FooBars.io, even if legally/administratively the two situations were equivalent.
Some places attempt to solve this by saying that you can use whatever vendors you need to solve the problem but often the best place to solve it is the other department since the solution is infeasible without them.
The problem is that often the solution is infeasible with them because the guys who made them good have left and now you only have the lameos.
As to why they can't operate as independent business unit? Well, there's the Theory of the Firm to tell you why that doesn't work often.