Maybe those who post are afraid that (as on some sites) the amount of space allotted is limited, and that their post will be cut off mid-way through.

I've been on sites where I just start typing away, only to find out after I hit the "reply" button that the entire message would not show up.

Note: even while posting this, I can see as I type that I am writing 3 paragraphs, and inserting spaces between each. However, those spaces do not appear in the preview.