1. In order to get a job, a person need to quit one. If there is a risk (in form of trial period), why to accept such an offer? As result, best candidates pass, or at least stick to existing positions longer. Or pay raise must be more significant.
2. Reputational harm: read about Amazon's hire-to-fire practice. And try to guess, why a lot of people do not consider employment with Amazon.
3. Work harm: any new employee is investment. It has to be onboarded, and it is a heavy load for the accepting team.
Of course, it heavily depends on the job market segment. I know software eng. only. If most candidates are unemployed, trial period could be a blessing. In the current market, why would I even apply (and spend my time on the interview?
Maybe getting sued if they fire candidates fitting DEI or other protected class?
If anything the risk is for the employee. But if companies stop trying to be Google and stick to their actual level of prestige, employees wouldn't have to do a multi week/month interviewing marathon just to get a foot in the door, and it balances out, easy to get your foot in another door.
I don't know if it's cargo culting, ego, or what, but non of the hiring practices so many companies carry out make sense. Pick top 10 resumes, spend 2h interviewing each, offer 3 month trial or at-will hire/fire your top pick. Saved you 50-100k in recruiter costs.