Not defending those griefers, but you have to understand they were in the game since 2001. Some people only played RS classic to slam doors on players. Some people on RSCR "merch" by spending all their money on a niche resource and jack the price up ten times because they get the same sort of fun out of it as the door slammers, even if they don't earn much doing it.
As for those people with 'smurfs' spread out every 4 levels or so enabling them to kill afkers who are literally any level in low level wildy, I do agree it's somewhat scummy, but what can be done about it? Making that many characters means the player practically lives inside the game. It's not illegal, even if it's scummy. They have a fully geared hero from every single level at 4 intervals from 5, 9 all the way to 123.
This is a very small community though which makes the griefers stand out. Most players don't bother doing that. However if you have those 10 griefers here doing these things (who have been doing it in RS for the last 20 years) and we have 60 players online at any time, it seems like a ton of people are doing it. But it's just a small core group of trolls.
edit -I wont bet money on this, but I *think* a mod will teleport a constant door slammer out of your way if you message them. They are obstructing your gameplay.