One thing is having markup in a place where usually you don't write more than a few lines, another is having it in a big document.

One thing is having it a free forum, another is having it in a product you pay for. One thing is having to use it, another is to actually like it.

And, as far as my experience goes, most of the posts in forums and the like are plain text. Not to mention that a lot of these forums are targeted at already somewhat technical people.

EDIT: And, on a personal note, markup even annoys me, even if I'm a developer.