> I gave them every opportunity to prove to me they could do the job (or learn at least to) they were hired to do, instead they cheated their way through the pip, passing others peoples work off as their own.

I feel like this is the problem with PIPs. From the managerial side, there is this good-faith expectation that a poorly-performing employee will snap back into shape once put on-notice. For people that are chronically incapable of certain tasks, this is a deliberately bad-faith expectation. And while it's not particularly common, it also stands to reason that a well-performing employee could be judged by unfair metrics or assigned an impossible task. So now everyone feels wronged. It's like a minimally-viable abstraction for making a firing appear natural and documented.