Why not?

I'd like to see a fully functional Lo Fi skin, especially since it is the skin for us iPhone users.

Is this limitation just because you haven't gotten around to writing such support to the Post form or is there a fundamental limitation here (I can't image why it can't be done).

You cannot manage attachments using the Lo Fi theme

The lofi skin was intended to be a NOT fully-functional skin. Basically, it was designed to give you access to the basics so you can quickly check over your site or read/post topics while on the go. If you want access to all the features, why not just use the full skin?

That said, Rikki has been doing some work on the lofi skin lately to update it significantly, as I understand it. No idea when or if his work will make it into the main branch though.