1) His lyrics are pretty simple...It's not like he is writing "Poet Laureate" and "100 Bars" in his head...
2) He does MULTIPLE takes in the studio...So he may not be writing on paper, but he is "writing" on a track...
I have noticed Jay Z's lyrics typically have one good punch line his fans will swear is the "holly grail," the rest of his rhymes are just random rambling that sound good to the people that love his "flow"...