Sigh-Fi

The real answer is that it's not as interesting to film and write a story around. In cars, they're louder and faster. You can set up scenarios for injured passengers and gunners. You can build up tension with zombies smacking the windows.

Bikes are too practical for action flicks and harder to mount cameras to for 3rd or 1st person chase shots. Not impossible, but also not something that relates as much to the general carbrained audience.

Some of the replies here speak of difficulty for spare parts. But it's pretty easy to stock up on basic components and repairing is way easier than a vehicle. Also, you don't need to look for fuel constantly. In a post apocalyptic world, fuel would be quiet scarce without a steady stream of production and would likely be more useful for electric generators or heating in winter. Vehicular travel would have to be hauling lots of cargo or people to be worthwhile to burn all that precious fuel.

That being said, it's entirely possible to make a damn good zombie flick with bikes being a more prominent mode of transport in certain settings. New stunts and different terrain.