I would say, "Maybe they're tone deaf", but "rap" doesn't have a single damn thing to do with actual music anyway... so it's gotta be that their aim is to annoy as many people as possible... just like every other "rapping" idiot.
Most rappers are people who can't do music, yes. I'm no fan of rap, that's for damn sure. Yet I have to admit, a few rappers do have talent, and I enjoy listening to what they do. Eminem, Grandmaster Flash or De La Soul come to mind.