Shortest of the answers, that land is NOT abandoned simply not used, big difference. The land could be held by the city, development company,offshore owner just for a start.
Bylaws most likely do not allow for discharge of any thing vaguely resembling a gun, including bb, pellet, bows of any type and even nerf in some places.
Even assuming that the city would allow you to use airsoft in that manner good luck getting the property owners permission if you can find them.
Lastly zombie games are fun for the shooters and lousy time for the zombies unless they are very well planned and thought out. Think weeks not hours of actual planning and writing.