Why do a lot of people in 15|10 format play so fast?

Disable chat. Enjoy the game. Beat them with the extra time.

You don't have to worry in the slightest. Use as much time as you want, those who want to play fast should play blitz games, not rapid. That's what setting a certain time control is for.

There's lots of reasons people play faster. Some players know they panic and crumble in long end games when low on time so they avoid that at all costs. Some play blitz pace, but then when the position gets complicated go into 'puzzle' mode. And a lot are just impatient, especially the ones complaining about you using your clock.

Either way, keep improving and around 1200-1400 you'll see a lot less of all the above.

Thats poor behavior on their part, you have every right to use the time you've been given.

What do you mean ? A rating of 1000 is already above average. There are players with rating 100 on this site.

If they are impatient, why do they play 15|10 knowing that in this setting the opponent can take much longer ? In addition to being futile ( and rude ) , complaining is also sensless in this case.

I want to play 15/10 but there are many unsportsmanlike players. They abandon the game with plenty of time. They should resign.

If the player resigned I can play a new game. As long as his clock has time, I can't leave the game. In 10/0 I wait maximum of 5 minutes. In 15/10 waiting time can be as long as 10 minutes. If I leave... I lost by disconnection.

Nah, having to wait and watch paint dry isn't fun. Playing is fun.

Many players are used to playing blitz or bullet games and carry over their fast playing habits to longer time controls. They might find it difficult to slow down and adjust their thinking process.

People do this in 30|0. Just tell them to play lower time controls or mute them. You have every right to use your clock as others here have said. Salty tears always taste the best.

Good thread. Same problem happens in 10/0 games (but I've never gotten the rude comment, that is just weird). About 1 out of 4 times I'll get an opponent who makes their first 10 moves in under 5 seconds (playing a system and/or pre-moving) and first 20 in like 30-60 seconds. By move 30 I'll have like 5-6 min on my clock and they still have like 9+ min. And usually they win. If we make it to an endgame, I'm usually under time pressure by then. 
Because they're basically taking time off your clock. A 10/0 game where your opponent only uses 1 min is more like a 5/0 blitz game. My opponent might make a blunder or mistake but since I'm under time pressure more analogous to blitz, I often miss it, as often happens in blitz around 900-1100. 
Perhaps these people play so fast because they're impatient or they're just accustomed to playing blitz, or they really want to save a lot of time for a drawn out end game. Or it is a tactic. And it works pretty well! 
So the simplest solution seems to be just play longer time controls - like 15/0 or 15/10. The other solution might be to play unusual openings that trip them up, or memorize the countermoves that mess up system openings.