This is what the MEMO key is for.
The concept is already present in the underlying technology that powers steemit (it is called graphene, conceived and developed by our resident genius Dan Larimer.
I am not aware of the exact reason of why steemit.com does not implement the functionality right now, it could be as simple as others things have been of more priority to the team.
To illustrate, here is how it looks when I start my bitshares wallet (which, as you may recall, was the first blockchain powered by graphene), with some details blanked out to protect privacy:
ORIGIN_ACCOUNT sent XX.YYYY SOME_TOKEN to DESTINATION_ACCOUNT
Unlock your wallet in order to see this memo
X months ago - XX.XXX BTS
The memo does not show up, because the wallet is not locked - only the owner of the memo key belonging to DESTINATION_ACCOUNT can ever read the message.
Same concept in steem!