Other people have mentioned the logistical/financial problems this incurs, but one other important consideration is that you are competing against other companies that don't do the trial-to-hire process and can thus offer more stability. And to be honest, given the state of the software engineer market (in tech-heavy cities), you already get to spend a couple of months trying out a company and then leave if you want. The difference is that the employer isn't similarly able to get rid of you after 3 months (it's too short a time to evaluate a new employee, the company has already invested heavily in onboarding costs, it's much slower to backfill a position than it is for a software engineer to find a new job, etc).
