They're actors. They're trying to create an illusion. In the Lord of the Rings movie, Ian McKellen plays a wizard. You think he goes home at night and shoots laser beams into his boyfriend's asshole? Tom cruise is a midget, but he plays guys that are normal size in movies.

Mac

Charlie: I'll tell you what. I'll take him down to the locker room. I'll lather him up real good. I'll strip all these silly clothes off him. I'm gonna clean him. Sparkling clean. Brand new kid for you.
Principal: No, please don't bathe the students.
Charlie: You're right. He's a big man. You can bathe yourself, can't you, Rich?
Richie: Yeah, dawg.
Charlie: He's bathing himself and I'm watching.
Principal: I'd rather you didn't do that either.

Charlie: Principal McIntyre, I recently starting mentoring one of the kids here.
Principal McIntyre: Really? That's odd, because you're a janitor.

Mac: A lot of great actors have done blackface.
Dennis: There's countless examples of very classy actors doing black face. We got the great C. Thomas Howell in Soul Man. We got the Wayans Brothers in White Chicks. That was a very tasteful example of reverse blackface.

Dee: Did you get that orange out of the garbage?
Charlie: I sure did. Can you believe that? Someone threw this away? Perfectly good orange. They were digging at it a little bit and must have given up on it.
Dee: Don't eat trash, Charlie.
Charlie: I'll eat what I want to eat.

It's Always Sunny in Philadelphia Season 6 Episode 9 Quotes

Mac: A lot of great actors have done blackface.
Dennis: There's countless examples of very classy actors doing black face. We got the great C. Thomas Howell in Soul Man. We got the Wayans Brothers in White Chicks. That was a very tasteful example of reverse blackface.

Dee: Did you get that orange out of the garbage?
Charlie: I sure did. Can you believe that? Someone threw this away? Perfectly good orange. They were digging at it a little bit and must have given up on it.
Dee: Don't eat trash, Charlie.
Charlie: I'll eat what I want to eat.