I think it's safe to say that this 'comes with the territory'. Airsoft is not a big business comparatively speaking, so you combine the fact there's not much money in it with the other fact that we're running around with 'guns' and throwing fireworks and hence need to be away/segregated from anyone not actively participating, then the sites are generally going to be in locations where nobody else is regularly travelling to. Unless someone sticks to the rare examples like Precinct 13 and Z-Mart that are actually in the middle of a fair sized town, then having a car and license is (close to) a requirement for entry for the hobby I'd say.
First time really thinking about it, but I'd almost say that if you're new/getting in to this and can't drive or get lifts, then either put the time and money in to driving before even getting eye pro and a gun or buy one absolutely barebones loadout then hold off further purchases until the driving is sorted. Bearing in mind of course that some people who've already commented here don't fit this category and do use public transport, so it clearly can be done. Personally though I wouldn't ever play if I had to spend hours on trains/buses and then potentially hike a long way with a heavy bag; then do it all again on the way home. Not knocking that method, it's perhaps the only option for some folks for various reason and the effort is impressive frankly, but not for me.