It's a matter of the basics of form, children. Bona fide songwriting requires a set of dimensions utterly lacking in rap: structure (in the Aristotelian sense--meaning beginning, mounting tension, climax, denoument); structure (in the chromatic sense); melody, a dimension of limitless permutational potential absent from rap; and the interactional dimension of melodic/lyrical con-fusion.
Typical song structure: A (intro); B (verse); C (chorus); B, C, D (bridge), C, maybe C again, etc....
Typical rap: A, A, A, A, A, A, A, A, A, A ... ad nauseum, "A" representing a looped sample of some pretty damned great Motown classic.
OK once again not all hip-hop is created from samples. As was mentioned before many different hip-hop groups have very talented bands that write their own original material. Clouddead, Dalek, and Blackalicious just to name a few, and they follow the same verse/chorus/verse structure that any rock band follows. And Even those that don't have their own bands have producers that use studio musicians and create original works just like the Motown artist you appreciate.
To put some rapster on the same level of a true songwriter is to do a great disservice and to show a lack of appreciation for the spiritual efforts of the person who bothered to learn how to play an instrument, tap into the art of songwriting, and create something original and indelible to contribute to the world.
Once again it's not just a "rapster", their are many bands that work together to create hip-hop music or it's the same situation of a motown artist who has studio musicians. And since when is poetry not original, indelible, and requiring of spiritual efforts. So poetry isn't art on the level of playing a guitar?
Now, mind you, I know that not all rap is the "typical" variety I illustrated above, but I have no interest in stuff like the Roots, whom people treat like Isaac Newton for learning how to competently wield organic instruments. Big deal; it just shows how much the bar has been lowered by rap for the demands we place on musicians.
The musicians in The Roots are as talented as anybody else that can play a instrument you moron. You don't think they can play at the same level as any rock band? You make it sound like everybody's so proud of the fact that these idiots learned how to pluck a string. Get a clue