Why is Runescape and Jagex's games so addictive?

Because it's fun! =D lol.

They made a very entertaining, fun, enjoyable, online game where there are so many things to do.

The fact that there are so many skills to choose from is one way that you don't get bored. If you get bored of a skill, you move on to the next which makes it so addictive etc .... coz it's SO FUN! XD

Also, it is an online game which allows you to chat to everyone else in the game which keeps you occupied and not just sitting and waiting for fish to be caught, bows to be fletched / strung etc.

Jagex are obviously experts and know how to make a great game! ;)