The main reason I have is that I just do not like the thought of losing items that I've worked for RuneScape Gold days or weeks on. While farming Elite Clues, I've been trying for the Chaos Elemental pet dropped and I'm currently at risk of losing 6k worth materials. I'm not worried if anyone tries to PK me or something similar. I'll let them drop the ball on me because I know that I am taking only small risks.
Another issue is that my account is just not built for PvP. I'm currently at 136 combat with three combat based 99's. There are three kinds of magic: Constitution, Defense, and Magic. My skill stats are all over 70, mostly during the 80's however without anything 90+, other than 99 Fletching (which doesn't help when playing PvP).
I'm able to attack anyone who has full 99 combat stats. While I may be able compete as a mage once I'm able to benefit from potions, beasts of mass and other types of weapons and armor, I'm outclassed.
I don't mean to be negative, but this is something that I am not too upset about. If I were to put in the effort to make my account competitive I would do it however, I don't think of the financial gain that comes from it. If I PK someone, it could net me, if I'm very lucky drygores.
Let's say I spot somebody who has been skulled, and I strike them by praying. Then it happens. They will likely outclass me at 2007 RS Items chaining since I'm not a pro at this. I'd rather have 90 minutes playing KK with my buddies and kill it 30x in the span of 90 minutes, and get my one drygore. PvP has the only part I love. But, PvM is so good that I can't imagine not playing it. It's a great question, and I would be interested in hearing other answers.
The Wall