Since it first existed. "Normal" singing is just spoken voices also, only the words are stretched out to last longer. Think of rap as just shorter notes. Short notes are music too.
Only the songs about sex, rape, and drugs. That's just generalizing. All genres have songs on those topics. There are so many rap songs about so many different things. It's quick singing to a beat.
Eminem is 900000004x better.
Lil Wayne is good when you can get past his voice and pay attention to what he's actually saying, in relation to the musical notes.