Being able to delete your own posts would open up the possibility of causing shenanigans by posting something, especially something other people disagree with or something inflammatory, and then removing your post after people have replied to it. This is an annoying troll gate that can be and is used on several forums as well as youtube.
In regular use of the forum, cases where the post actually should be deleted, because it was and accidental double post or something, are rare and thus can be taken care of by moderators easily.