That's the way I see it too. But my experience is: They win, I say 'good game' they say 'good game' too. Then I win, I say 'good game' and they leave, trash talk or say nothing.
Yeah, that happens quite often. I don't like saying gg after every game though, it feels stupid. Usually I compliment players when they're about to leave or send them a message/post on their wall here.
There's no DOA scene in my whole country, so online is all I have. I'd love to see how different it is to play the same players offline, but there's almost zero chance for that.