Because it will take the replies to your posts out of the context.
Not quite correct. A member IS allowed to remove their posts - and this
does result in other members' posts losing context.
HOWEVER a member is NOT allowed to remove the
very first post of a thread. This first post is an anchor point for the whole thread and if it was removed, then the whole thread would disappear.
Having such power over other members' posts is not allowed.
You can edit the first post down to a single character and you can do the same to the title - but that first post MUST remain.