There's definitely legal reasons, however the main reason we don't is people tend to argue. "You don't have enough experience in ___" -> "Here, look at these 4 projects I created! You didn't give me a fair chance by asking about them!". Or "You didn't seem interested enough" -> "I was just tired!"

I've found most of the time, unfortunately there's no good in giving details. I realize people genuinely want to improve, but most of the time the reasons we don't hire someone aren't actionable.