Players who play fast in longer time control games I see it all the time on the net. Some players play, for example, a 15-minute game with a 10-second increment, and they finish the game with more time than what they started with! I've heard players doing this even in OTB tournaments- games that could go on for hours on end.

Is it because they are perhaps too impulsive? (And if so, why not just play bullet?). Or do they like to just use their gut? Is it also a bit an ego thing, as in, "I can beat this guy even if I don't think at all!". Or is it also a part of their strategy, they want to get their opponent in time trouble?

So what are the possible reasons why they play so quick?
As I commented, there are several reasons why people play fast, among them:

One of the reasons I often see is fear of time trouble. Many amateurs are not really aware of whether the time they have left will "be enough", so start to move way too fast early on. They won't feel confident to play faster games either for that same reason.

It could be the case that your opponents are using your time to think if you take too much time, so when it's their turn they've already anticipated your move and figured out how to answer.

Other players want a challenge, so they will play a game in 5 minutes against an opponent who has 15.

The list could be infinite...