The whole issue is to identify a "known performer" in the first place.

Otherwise, that practice being more common means people can leave their job for weeks here and there no questions asked ("I'm going to work for our main competitor for 3 weeks" won't fly if it needs approval), and come back without impact or penalties on their performance.

That would be ideal, but we're so so far from that. Even getting people to properly take their parental leave is still a fight.

I’m sure some people would accept being paid to try out without the company having any commitment to hiring, but those are probably not the people you’d actually want to hire. If a company can’t commit to hiring then why would a confident professional bother with the company? A probationary period seems like a reasonable compromise where the company commits to the expense of onboarding while still having an escape if it goes poorly.