I agree, rap is full of clichés, but that is where the genre falls down. It doesn't have to be that way.
Believe it or not, I was a fan of the early stuff. Back in the late 80s and early 90s, there used to be KDAY 1580AM in Los Angeles. (Later resurrected as a FM station.) It was the only station playing rap in LA at the time. I used to listen, as did a lot of my friends. Back then, it really was fresh and new. Something different and a lot of fun. Then rap just stagnated. You kept hearing the same things over and over and over. I lost interest.
Fast forward to today, and they're still recycling themes from 20 years ago.
Sampling was new and interesting at one time, but it's since turned into children's music.
The children's music genre takes a catchy musical phrase and keeps repeating it with lyrics that are frighteningly similar. Rap does the same thing. It runs a catchy snippet over and over and never progresses. Contrast that to classical. In classical, you'll get a musical phrase a few times - enough to establish it. Then the composer will turn the phrase upside down, switch keys around, complicate the phrase and send it to different sections, introduce a countermelody with a variation, and much else. Listening along to what the composer is doing is what makes the music interesting. You can listen 20 times and then notice that the strings are doing something you never noticed before.
It's hard to do this in other genres, but you'll find development in even standard rock songs. The Beatles are particularly good at this. After the first verse and chorus, you'll find them pulling a bunch of sneaky musical tricks as the song goes along. Not as complex as a Beethoven, but the intelligence is there. Jazz musicians will do this, too, and you'll also find it in many genres.
But rap is just pure repetition. You hear one idea - and sometimes it is a good one - but then it goes nowhere. It's like those SNL skits that are funny for the first 30 seconds, but then they'll repeat the same joke for another six. It gets boring, fast.
I blame a lot of this on the lack of music education in this country. If you grew up playing an instrument and having a teacher explain to you how what you are playing is developing through the piece, you come to expect development instead of repetition.
One of few rap albums I still listen to is "3 Feet High and Rising." De La Soul did a great job with it. Even then, they saw the clichés and steered around them. The samples were interesting and revolutionary at the time. The album is tied together with themes and touches on them in new ways throughout. Too bad other artists didn't pick up on this and continue the trend. If they had, I'd be a huge fan of the genre.