Futuristic Linguistics

Rappers are pretty amazing. Modern-day rappers can perform linguistic stunts that would baffle the old bards and poets, I'm pretty sure; I've seen numerous examples of rap that contains real literary quality, and many of the best even improvise it entirely.

Question is, why does nobody in "higher literary circles" pay attention to this? I've seen rap lyrics that I'm pretty sure would make Shakespeare impressed (if he could understand them) because they have a nice rhythm, good rhyming, and are peppered with similies and metaphors. Take a look at this, for example:

"When I punch ya, I rupture all your ribcage in a rage,
and I turn you into a cartoon too and erase the page."

The above two lines have some pretty good rhythm, and we've got at least one pretty advanced metaphor ("turn you into a cartoon and erase the page") and one somewhat more crude one (assuming the rapper isn't literally threatening people with crushing their ribcages, which rappers aren't prone to do). This isn't bad poetry - it's pretty advanced poetry. Its message might only be "I am awesome", but poetry has never concerned itself much with message - what makes poetry poetry is that it has literary quality, and it can't be argued that rap does.

Here's another example from some guy called John Cena (he's probably famous, I never heard of him before):

"Cause I'm ill like a sick metaphor,
got the crowd shouting Cena cause they want some more
[...]
hold your hands up, hold 'em high,
that was a nice diss, but it was a wack-ass try."

Not perhaps remarkable mastery of language, but he made this up on the fly as one dude in the audience insulted him. And still, the guy manages to fire off "ill like a sick metaphor" which is a pretty advanced similie and something that, I'm quite sure, literature professors could spend a few hours poring over its hidden depths had it appeared in some musty old book on poetry.

Why aren't rappers given the credit and respect they deserve? They're doing massively impressive things with their language, sometimes in advance, sometimes on the fly as they stand there. What with the speed a rapper usually keeps up, this makes many of them way more advanced than many poets. Granted, the message might not be so deep, but should that really matter? If an artist only paints angry dogs, but does so amazingly well, is he any worse of an artist just because the motive isn't deep and inspired?