The backstory/timeline of the game is not set in stone (or even Jello) so it's difficult to tell how much time has passed since the zombie apocalypse and if it was a massive takedown, or if areas of the U.S. continued to function under siege for years after the first infection.
Gasoline is only good for a couple years. If we're finding gas in cars or gas stations, then it's been less than two years since Z-day (/salute Z'ha'dum!).
Logically we'd need another source of fuel if there was to be a future. Of course, game-balance is a thing too.
-Arch Necromancer Morloc 💀