Maybe it's because I'm an imcompetent DM, but when PCs reach about 11th-13th level, I have a lot of problems dealing with it, so the campaign usually doesn't go much higher than that.
Also, I suffer form ADDD&DC (attention deficit disorder for d&d campaigns). I end a lot of my campaigns because I get bored and want to try something different, which never happens to be that different, and so I get bored again relatively quickly.
As a player, if I'm playing with a DM that can handle high level play, I don't mind playing. I'm currently in two campaigns that have been going on for a while (3+ years) and we're at level 12 (approx.). We'd be higher if we hadn't died multiple times each in the last few sessions...
AR