> once you hire a wrong candidate it is extremely difficult to fire them, even with at will employment contracts

I live in a country where the probational period is 6 months (Germany), so the law says people can get fired without cause or justification, just give them 2 weeks notice or 2 weeks garden leave.

Even with that, it's a very arduous process to fire people. Even in cases where they repeatedly refused to show up for work, I've seen HR departments asking managers to "try harder". This only makes both the employee and the manager suffer.

The reason for that is simple: HR departments are extremely inefficient machines.