Because rap isn't music in any classical sense . . . it has no creative melody, tune, or musical complexity, it is, in short, lyric driven and rhythmically boring.
It is entertainment, yes. It is a particular genre of poetry with lots of obvious internal rhyme, but it is simplistic.
I suppose it's cultural, but the aggrandizement of those who espouse the lyrics contained in rap is off-putting to those who have studied and appreciate the effects music can have.
What do people hate on rappers? Likely because they make so much money and are held in high esteem by many for doing so little . . . Of course this is just my opinion as a trained traditional musician.
I personally don't like a lot of rap because of the message it sends. They are very offensive and the lyrics are not how I want my kids to think. Guns, drugs, sleeping around, and mistreating women is disgraceful.