some dudes have "signature" bits, comics get big from being able to do a full set.
The reason that comics get so pissed about this is that every show they're trying out one, two maybe even a whole set of new bits to work with, then going back and building on what worked and either shitcanning or rehabilitating what didn't. A guy that gets big while filling in gaps in his act with other people's stuff is a big fucking deal to people who are legitimately "grinding" as you say Mencia is. When he's getting big, he isn't getting his. He's getting theirs.
It's the same reason blues musicians got pissed at Led Zeppelin, the Stones, or Elvis when they first came out.