My bet is it is not the day after, but some time since zombifications has already passed. At the very beginning those that could took the bikes and run away, so that is why there is no bikes. Then those that stay started to defend them self and gathering a lot of ammo and guns was common and that is why there is a lot of it today. On a side not: once I have found two bikes at one place. But that was one and only incarnation. Never before or after I have seen a vehicle of any kind.