"If only there were some sort of middle ground between cars and easily spooked animals!"
Why don't they ride bikes? Did all the zombies eat them? Did the nukes somehow specifically target bicycles but miss all the cars? Bikes are cheap, fast and easy to maintain, plus they require no fuel and they're freaking everywhere -- literally the only reason we can think of for why they are never used in these films is that they would look kinda ridiculous.
Bikes: Worse than being eaten by a zombie.
In The Road and Book of Eli, the protagonists spend pretty much the whole movie walking across hostile territory and never so much as consider looking for some bikes. It's like they never even existed. And before you tell us that Eli wouldn't be able to ride a bike due to his condition -- if you can aim a bow and arrow and win a machete fight, you can ride a damn bike.