Is the concept of a probation period not universal? I mean if a new hire is terrible, it will show in the first 2-3 months, and during this period they can be fired without any repercussion whatsoever.
Most bad hires can be rooted out in a few months, yes... but a not insignificant number of them can take years as well. Also many, if not most people, simply stop learning and allow their skills to stagnate and that can become a liability after a couple of years.