It's not always that simple. What if that person quit their last job to come work here? Quit and moved across the country? Gave up other promising opportunities?

I'm not saying you can't fire people, but unless a trial period is explicitly agreed upon beforehand, there's an expectation that a job will last more than a couple of months if the new employee is putting in real effort. I don't know where to draw the line, but even an employee who isn't a dick might sue if they rearranged their life only to be let go two months later; I'd argue that they also deserve to win.