Once proven that a candidate is able to learn and adapt to the job, they should be moved to a full-time position, with pay above what is currently being given to that role. If they aren't excellent, let them go. As a company you want to advertise this promotion experience so that people who know they will pass this test will feel more comfortable signing up for a "lowly" temp position.
The top 5% or so of candidates might not take that offer, but you can leave a hole for exceptional candidates based on past experiences and proven success.
The problem I usually see with most people is, they think they already learned it.
Many people I met thought they had good knowledge of human nature, even the ones who were failing constantly in different parts of their life. Even the ones who where obviously hanging out with toxic people all the time.
Everyone thinks, even if they don't know much, at least they know that, but I think it's one of the hardest things to know and nobody really puts in real work to get good at it.
Another problem with hiring is that in reality people will be flexible and adapt to the traits of a new person in their group, if that person has something valuable to offer.
Nope, Hiring is basically almost random no matter how much energy or tests you put in it, you can never know someone well until you actually work with them on a daily basis.