But how can we know that we picked the right person for the job if we aren't at the very least absolutely sure the alternatives were worse?

The human mind resists accepting decisions made without a proportional amount of effort put into those decisions. If we only get one candidate for the job, we will either hire them and deal with constant nagging doubt, or we will lower our standards to get more options we can reject to feel good, but at which point we've now created false hope in the remaining candidates and an illusion of more opportunities existing than actually do.

Humans are messy, and so everything is always fucked. Even when it's not, we find a way to make it so.