As in any genre there is a wide range of talent and musical ability among rap artists. The record industry is really good at printing one hit wonders and then burning them up. Those are probably the guys who are making the songs you are referring to. This is not unique to rap, it happens in pop, alternative, rock and roll, heavy metal and any other genre. I could easily say "metal is just screaming" or "alternative is all the same crappy guitar chords" and I would be kind of half right some of the time.

If you take a look at some of the more accomplished artists with staying power you will discover thoughtful lyrics and complicated music. Jay Z has been getting more and more musical over time. The American Gangster soundtrack was an explosion of funky horns and beats that really set the tone of the album to that of the film. Death of Autotune doesn't have an electronic note on the track. This is not to mention the vast pool of talent in the underground hip hop and rap scene, which is something you should really investigate if you are going to give the genre an honest examination.

If you don't care for rap music that's fine, but saying "it all sounds the same" just makes it obvious you don't ever listen to rap music. What would you think of me if I said all classical music sounds the same?