There's even more overhead on the people being provisionally hired.

Yes, sometimes things just don't work out. But, if someone quits a job and maybe relocates, that's a big personal cost. It's just the way things work in some limited contexts (e.g. professional sports) but it's not and shouldn't be the norm.

I suppose you can give a huge sign-on bonus with no claw-back provision, but that's never going to happen in most cases.