Reason is that if you delete posts it has an effect on ranking and badges, and in case of an incidental deletion (where you may want your post back) it is not possible to restore any deleted posts, so the system does not allow by default to do this.
Due to the same reasons, moderators should not delete anything as well, but they can still take care of such cases if you edit your original post and leave a message for them.
So having ranking/badges takes precedence over proper forum etiquette? I think I prefer being able to delete my own posts knowing that it will reduce my ranking/badges, than having ranking/badges in the first place.
Isn't the main purpose of this new and "improved" forum to help the Archicad user community reach the proper information all in one place? Having to sift through a lot more information that might have been off-topic/accidental like @Lingwisyer said, just because of how it affects the rankings seems a little absurd to me. Other platforms that have post-based rankings like Reddit manage this without a problem.