They are more visible by posting everywhere all the time.
The post count is a status symbol:
The post count and title are what you always notice along with the nick and avatar. It is also a major part of appearance, just like avatars, maybe sometimes even more. Higher post counts do make an impression that the person is somewhat more "reliable" or veteran or something like that. It drastically reduces the chance of one being thought of as a troll or a purple llama that has wings and flies around chewing innocent kittens.
You don't get thousands of posts without even trying to be a one with a huge post count. That's just a fact. No-one is trying to keep theirs minimum, but some people just care and some don't care about what their post count is. Most of those people that do care certainly won't admit that.
Post count can be competitive. This is a gaming forum anyways. And basically, if you are here very often, gaining a large post count is just another thing to gain, to accomplish, to rejoice.
It's something some people can be proud of. If you've got, let's say 5000 posts, you have thought something along the lines of: "hey, that's pretty cool, fellow escapists will look at me differently now". That is just a part of being a human.
I don't like the fact that post counts can affect the way one is noticed. If the post counts weren't visible, I'd assume there would be some improvement in the content of posts and a somewhat noticeable decrease in the amount of posts overall.