[tbrownaw]

Being the bad guy is no fun.

People are not fungible, and depending on internal politics and procedures hiring probably only happens reactively instead of proactively. If you have to fire an under-performer, you're now short-staffed even worse until you can get another job posting approved and someone hired for it.

Lost productivity from being too picky is less visible than lost productivity from telling someone to go home.

Management is often seen as a career progression from non-management. This means a lot of managers - and especially the newer and therefore lower-level ones - are still figuring out wtf they're doing and aren't (yet) very good at it.