People play runescape for the same reason they play mir. Its a game they got good at, and know how to play, and their years wasted on there has given them some sort of reputation and they can "pwn nubs".
The thought of moving to a new (and better, because theres tonnes of games better than mir) and being bottom of the food chain, and not being able to play properly terrifies them, so they sit playing the same ****, killing the same mob for days on end because at least they know how to do it.
Runescape is bigger than mir because its free, and when you compare the £7 a month or so mir costs, to other games that charge the same, you realise just how much they can rip off a bunch of faggots that are not only terrified of the real world, but also other gaming worlds.