Because it gives you an actual reason to build up your bases defenses, versus building up your bases defenses for no reason. You'll care more about where cameras are placed, how your troops are equipped, what animals you have, etc. And, the most important part, it gives you the opportunity to invade other people's bases, which essentially gives you a never ending supply of constantly changing sandboxes, which leads to high amounts of replayability.
So basically, the fun is in the fact that it's more game to play, therefore more fun to be had.
(And honestly, something tells me that other players won't actually be able to screw you over that badly. Like, ok, you took one of my trucks, guess I'll just get 5 more from the next single player mission)