Super rich who are famous, probably. It's also a matter of convenience: you don't want thousands of people with "great investment ideas" to approach you constantly, and the security will not only stop you being assaulted but also stop randoms from coming up to you.

But I do know someone who is merely an heir, who doesn't need to live with a bunch of security all the time, because he's not famous.

So probably fame has more to do with it than wealth. Not sure what you do if you're famous but not wealthy, I imagine that could be problematic.