This is the illusion of the selfish, of course. They know that they are like this, and so assume that everyone else is like this as well. Thankfully, this is not the case. We wouldn't even have a civilization in the first place, if this was the case. Self-interested is not the same as selfish. You can desire something for yourself, and still work together with others to ensure that they are rewarded, as well. It's why you can have things like pony farms where players DON'T bail as soon as their pony drops. Most individuals understand that helping others to succeed will help themselves to succeed, as well. Helping others to improve will improve the general quality of the player base, making it more likely that future instances will run more smoothly.
I fully admit to being an optimist, but I don't believe I am a naïve one. The horror stories you read on the forums should not be seen as typical in-game behaviors. When something bad happens, folks have the urge to post. That's all. Have you ever finished a dungeon and thought to yourself, "Boy, that was adequate, and got the job done! I should post about this on Lodestone!" Of course not. Heck, even dazzling successes rarely earn that honor. No, the forums present the game as a cesspool of unteachable n00bs and insufferable trolls because those are the incidents that give people the urge to post, not because those incidents are the norm.
My experiences in this game indicate to me that this is the case. I've played since ARR release, and spent a LOT of time in Duty Finder instances. I have run into my share of bad players and trolls, but they are a miniscule fraction of the total number I've encountered.
Edit: Also, note that when I say to "help people", I'm not insisting that you take them by the hand and tell them what to do. Simply being silent can be enough, allowing them to learn on their own. It will take longer, certainly, but it's still a form of help. Verbal abuse instills a feeling of frustration and depression; not at all helpful for learning. Rage-quitting renders them unable to improve, as well, since they can't proceed through the instance. You don't have to help them - but try not to hinder them, either.