I have never seen a truly funny celebrity roast. Why do they keep happening?

I watched the James Franco one out of morbid curiosity, because everyone said it was great, and even then it was just a long, uncomfortable, unfunny mess.

The worst part is when the target of any nasty joke has to sit there howling with laughter and slapping their leg while someone says a string of putrid shit about them.

I kind of understand the argument of "we like to see people on top taken down a peg," but that doesn't quite work in a lot of these situations. Because most of these roasts involve โ€” at some point โ€” bigger celebrities taking aim at smaller comedians and making fun of their lack of career, which doesn't work from a power dynamic perspective.

What am I missing? Why do these exist?