Shout-outs, swipes, namechecks and borrowed verses: Hip-hop's cultural memory and rock's apparent lack thereof (x-post w/ILM)

It's like this: someone around here recently defended (or whatever, I forget the context...probably ass-fucking) Lloyd Banks' lyrical l33tness by citing "cooler than the other side of your pillow" as evidence, completely unaware of the fact that he bit this from someone else (I forget who). Although, "bit" isn't fair; Lloyd Banks probably assumed that much of his audience would get the reference and give him and each other mental high-fives for being so fucking learn-ed in the Way. And there's absolutely nothing wrong with that.

My question, however, is this: why is the namecheck/shout-out/whatever so prevalent in hip-hop (and jazz, too...) and not rock? Or rather, why does hip-hop encourage and applaud the (re-)citation of obscure (and not so obscure) lyrics/choruses/etc. while rock avoids it? I mean, the only rock song I can think of off-hand (read: 30sec of thought, here) is Built to Spill's "You Were Right." I'm sure there's more (what are they!).

But still: why? Is it even important? Am I totally wrong? How many beers can I pound before someone says the R-word? What's wrong with this horse and why doesn't it move when I hit it with this stick?

Full disclosure: I haven't heard the Lloyd Banks song/lyric in question. I really like hip-hop, but have only recently begun to realize that it's something I would have gone apeshit for as a kid (I grew up in rural MN=no rap). Why? Because I collected comics like some of y'all collect rappers (and all the attendant minutiae). I love the casual display of deep and useless knowledge.

Anyway. Discuss.