Yes, I have looked. If you wish to sell 'protected' versions of books you need a licence, a minimum sales figure, and annual royalty to apple and a healthy percentage of sales. I already looked into this a year ago.
Overriding factor also is that a 'straw poll' showed 2/3 of people still wanted the printed version so they could write on it etc. etc.
As it is, we are pretty much at the minimum order quantity to get the pooks printed at anything like a reasonable price. If the demand for paper copies went down (it is never going to disappear) then it could become unfeasible to print them.

Maybe in the future the situation will change. At the moment there is insufficient demand for us to be able to consider other options or we would have already done it.