The rationale is that a bad hire is very expensive. Not just salary, but emotional and temporal investment. Let's say you need to hire a salesperson and hire the wrong one. You fire them after two months and now have to context switch into hiring again.

For small startups, firings have an effect on the entire company. Absolutely make the right choice and let someone go if you have to, but it still has an amplified cost across the team.

I personally take responsibility for the emotional cost on the candidate. Asking someone to invest themselves in your company and then letting them go is a rough experience for the employee. It is the manager's responsibility to not churn and burn through people.