Because hiring people that you think you'll probably fire is not a nice thing to do: people quit jobs or reject other offers based on it.

I was involved in a hiring decision once where we were on the fence but made the hire because we really needed to fill the position. The person didn't work out for all the reasons we expected, and we fired them ~3 months later. It was really shitty, and I will never do it again. It's counterproductive, and it's unethical.