Because you have to pay a ton more for a lot of digital copies of books if you're doing any kind of lending and you're competing with existing libraries. So you both have higher costs (e.g. you have to pay more for a digital file that may have DRM, be required to self destruct after a certain number of uses, doesn't allow multiple lends at a time, etc.) and your prospective consumers have a free option so convincing them to pay is going to be difficult.
The economics aren't there.