That's an interesting idea. I suspect the reason that hasn't happened yet has something to do with storing steemit data on the blockchain. The Steem blockchain has to reason about posts and their attributes (I believe), so I imagine that it holds all the data from steemit posts which may be a limiting factor for posting large files.

The whitepaper doesn't say anything about that, but I'm very new to Steem, so there may be some technical documentation that someone can point to.

By the way, I think that an embedded youtube video or other link is not so bad, at least for the moment.