Or that people are easily convinced that silly rituals are mandatory when they in fact are not.

Note that HR never asks if you'd like to have an exit interview, and just ask you when you'd like to have it, as if it's already a foregone conclusion. This same trick works a lot more than you'd expect in real life. Stating something as a foregone conclusion and redirecting the line of questioning is a pretty sweet strategy for getting what you want.

Well, that would be a reason. Not a very good one. The whole point of asking, I think, is to see whether there's actually a good one, or whether people just go for the reasons you describe.