Because it's hard to fire someone. Lets discount the emotional cost completely and simply look at this in terms of financial risk to the company.

Employee X is hired. After a month it becomes clear to the whole team that X is a bad hire. But how do you turn a subjectively obvious feeling into an objectively obvious reason to fire. Because if that person decides to sue the company after getting fired that's what you will have to demonstrate.

Just imagine how hard it is to quantify what makes someone good at our job? Code Quality? Team dynamic? These are really fuzzy metrics.

You could document how many times the engineer affected production badly and how much that cost the company. But to be realistic even great engineers do that. Heck I touched production at Google and decreased revenue by millions of dollars and it was expected that this happens. So you'd have to let him affect the company enough times to be obvious He's worse than your other engineers.

You are going to have to have documented enough clear failures to do the job to justify starting the process of firing. If you don't have a performance improvement plan then you might be able to speed up the process a little. However not having a performance improvement plan is both a bad idea and, I would imagine (IANAL etc.), could cause problems later on down the road in a potential suit.

Taken in that light 9 months doesn't seem that long at all.

In the US you do not have to prove that person was not qualified.

It is "at will employment".

There is no need to prove that employee is causing losses.

It's the employee who has to constantly "prove" to employer that this employee's salary is justified.

In theory this is true. In reality you will probably need said proof to prevent questions of discrimination of some sort.

That's why you have a probation period where it's all worked into the equation that you might be let go after three months if it's not working out.