I disagree with what your idea of proper hiring apparently is; to hire and test out a lot of people often is better than to spend extra time finding the right person. In my experience, a work environment that is a revolving door of often failing new staff is a waste of everybody's time, whereas a work environment with more carefully selected new staff that sometimes fails is only a waste of management's time. Such are the burdens of management.
I agree. This sounds like bad/lazy management, and a broken on-boarding process.
As a consultant, I've grown used to working around bad on-boarding processes but most FTEs aren't used to jumping into existing teams without being given a lot of knowledge. I can imagine tons of great people washing out of such a team not for any good reason but just because they aren't used to self-service on-boarding.
Having done a fair bit of hiring myself, I can't even begin to reliably identify gems. If you know of a way or can explain how multiple rounds of interviews can do a reliable job of identifying them, I'm all ears. I can tell the stinkers right away I think, but gems, no that's really hard. The best candidates I've interviewed (resumes, work experience, knowledge testing, etc.) haven't had any better luck becoming great team members than those with a mediocre interviewing quality.
Cause here's the thing about gems: They only work within their setting and can be created, with some effort and skill, right out of raw material.