Lots of reasons. First and foremost is they are doing these interviews to get to know you, not your prep company. Imagine hearing the same scripted answer/format day after day from people wearing the same color suit and tie. It get boring and frustrating. Want to see their faces light up, tell them a different kind of story, talk like you really want to get to them and they really want to get to know you, because they do.

Yes, prep companies to not teach you to give scripted answers, but it comes across that way in execution. It bores them and frustrates them, so they give you some rope to hang yourself with.

They know everyone preps and expects you to prep. They just don't like the product these companies are putting out. There is a reason they can always tell what company you used. They also don't like when their exact questions are leaked making them have to do work to come up with new ones. They would rather you just practice, be comfortable, and be yourself.