Why is it so bad to have a bad employee among the first employees ? I don't understand why it is that bad because it seems that if he/she is bad, you'll notice quickly and then can just fire him/her. Even here in Belgium where employee contracts protect more the employees than the company (at least that's the theory), at the starting period, firing can be done very quickly and I think it can be done even more easily in the US.

Also I think the best way to see if someone's good or bad is to put that person to work. This would e.g. mean that if you have filtered out candidates to 8 persons, you could get the one you prefer to work for 1 week. If your choice was bad, you try the second. In the worst case, you have made a very deep assessment of 8 candidates in two months. I mean that I don't understand the point of recruitment processes that take weeks with all kind of proxy metrics for how a candidate would fit in a very small team.

If you can't find out in a week that the person is bad, how can a recruitment process be better ?

Edit: I'm surprised by how the 1 (paid, in my mind) work week is perceived in the (so far) 3 answers below. It happens all the time to have phone interviews, HR interviews, technical interviews, possibly pseudo-psychological tests, and technical tests in a recruitment process. All that time without being paid (sometimes well before you know what the salary could be). It's all about trade-off, I'm not talking about ditching the selection process and instead hire-and-fire liberally. My question was really just about why is it so damageable to get a bad hire.