Recruiting good people is hard, and sometimes you make a mistake.
Soldiering on with an employee who doesn’t have the skills you need, and is showing no sign of learning them, can suck up all of your time. It is no fun for the employee to feel out of their depth the whole time either. Life will be far better for both parties if you can cut your losses and let the person move to a job they are suited for.
Legally this can be tricky, which is why a probationary period or short initial contract is a really good idea.
As mentioned in another post, my ideal employees are bright, motivated and fast learners. The useless people you want to get rid of are likely to be the opposites.