Well this game goes for a long time and if your doomed from the start why would you sit around for another 2+ hours dieing a slow death and waisting allie's ships?
Mind you if never been online, just singleplayer games that last far to long.
You keep playing because it's a team game and you try to do what you can to help your team, even if you're losing. If you're losing then you try to be as big of a pain in the ass to kill off as possible, consuming the enemy's attention and fleet supply points. You also try to keep your capital ships and fleet from getting destroyed for nothing and have them join your allies' fleets and try to help them out or use them to hit your opponents in the back, annoying them and forcing them to bring back their fleets.
In contrast, if you're playing an FFA (free for all) then feel free to quit whenever your want.
Here's another example, the large random multistar map (4v4). Oftentimes three teammates will start out in one system with one opponent on it and vice-versa, meaning that there are two 3-on-1's. Unless the three are noobs, the guy facing the 3-on-1 is probably done for (though he might be able to move to the empty star system). However, that very same guy facing the 3-on-1 can win the game single-handedly by expanding in his starting system, fortifying his planets, not losing his fleet, being a pain-in-the-ass, and delaying the three guys in his system from conquering him. If he makes it take long enough then his three teammates will have finished off the one opponent in their system and will be ready to head over to the other system and victory.
The point is that it's a team game and just because one player on your team might be losing doesn't mean that the team will lose as a whole. One of the most memorable games I ever played was a 4v4 where started in a bad spot and I was reduced to a single ship--a mothership capital ship. I had a great time managing it and attacking enemy planets as part of ally's fleet and we won.
