Because people (Myself included) refuse to carry dead weight around. If you have low gear/starter gear, pls do Kahrun's/TF/S1K/BM for ancient coins and try getting some gear and/or experience before going into higher-end instances such as RST/Kata.
If I do RST, Kata, or even S2K, I would like DPS classes to out DPS my templar. I don't know about you, but I care about my reputation as a player. I always make my own groups to almost every instance (Except Runa Normal). In this case, I don't want to be that guy who always takes randoms and fails instances almost every time. Sorry, but that's how most geared people act in this game. It wastes my and everyone else's precious time.
And it applies not only to DPS but also to the overall experience. Just yesterday actually, I went Sauro 2k with a random low gear ranger because it was late, and I some top quality DPS in pt. Guess what happened? Without saying anything, that ranger decided that 2k was the same as 1k and went to bug a boss. Thankfully I'm a decent cleric and along with my friends, we cleared all the mobs without dying. A couple of months ago the same situation happened with another sin, but that time we failed 2k because PT was under-geared and inexperienced, and the cleric wasn't able to get up and loci us quickly.
You're saying "WE SHOULD SUPPORT EACH OTHER", but how exactly do you support my PT by being a low DPS, low skill pile of pixels that we carry around? We do DPS/tanking/healing while you wander around and agro random mobs that you shouldn't agro, wipe your PT in Kata/IS because you don't know that you have to run away when Hyperion nukes you, try bugging Sauro 2k when you just have to stand and agro mobs around the boss, go panic mode in RST when a bunch of mobs spawn and more and more and more things that you mess up and let us clean it after you.
As @Haku said, make your own groups, or join a legion, or try making friends and gear up together. I in fact like helping players who are actually asking for it. If a player is saying that he/she is new here and needs help getting gear/experience, I'm happy to help with whatever I can. What I don't like is when I'm making PT to an end game instance, and someone keeps applying and applying even though I said that I don't need that person in my PT for various reasons.(gear, experience, have a friend coming)
If you're an active player and you come to me asking how Sauro or Danuar Sanctuary is done, I will be happy to explain. I will take you in pt, maybe ask to join Discord if you can, take a friend or two whos' also willing to show a new player how an instance is made and we will go with the teaching mindset. We will take our time to explain everything, each mob, each boss, etc.
And if you keep playing and getting better, I will be happy to take you forward into Sauro 2k/RST/Kata with me constantly. And when you have enough experience yourself, you can make your own PTs.
But please, do not apply to high-end instances when you have no gear nor experience to do them. You will piss off players and make a bad image of yourself.