My simple reason is that it's something that RuneScape gold I don't like to experience. I hate the feeling of losing items I've spent days or hours grinding for. I've been trying to obtain the Chaos Elemental pet drop (while working on Elite Clues) and I'm risking around 6k worth of material. I don't mind if someone tries to PK me or something similar. I'll let them drop the ball on me since I am aware I'm only taking an extremely small risk.
My account is not intended for PvP. I'm 136 combat, and have three combat-based 99s. My three areas of strengths are Defense, Magic, and Constitution. My stats for skill all go over 70, mostly in 80's. I don't have anything 90+, except 99 Fletching (which isn't very useful in PvP).
I'm able to attack anyone who has the full 99 combat statistics. While I may be able compete as a mage once I've got the benefit of potions, beasts of weight, and other classes of armor/weapon, I am severely outclassed.
This isn't to say that I'm complaining. While I could do everything to make my account competitive but I don't see it as a benefit to my financial situation. If I'm fortunate, some drygores could be my reward when I PK someone.
If I find anyone with skulls, I'll smite their prayers and, the most important part, actually kill them. They'll probably outclass my chaining abilities. I'd prefer to have a couple of friends sit down at KK and beat the game 30 times in 90 minutes. Then, I'll grab my drygore. PvP is my absolute favorite aspect. I love the excitement of PvP, but it's not the only thing I love about PvM. Great question though, I'm curious to buy runescape 3 gold hear the responses of others.
