Publishers sell books to libraries. A book can be checked out about 20 times before normal wear and tear makes it unsuitable for circulation, and a new book is purchased. Also, they'd buy multiple copies of popular books so multiple people can read them at once. Once those books have been read, they don't often get checked out again. Imagine Oprah recommends a book for everyone to read. High checkouts one month, low checkouts the next month.

For eBooks, the same sales paradigm as physical books has been replicated. Libraries purchase bundles of X number of checkouts, with a certain number of checkouts at a time.

You're right in that you absolutely could do as you say, technically speaking. But that would infringe on copyright. The rights holders have the right to sell them as they wish.

What makes eBooks different is that each time you lend an eBook, you are not actually "lending" anything. You make a new copy on your device. Even if you are prevented from accessing the original copy, it's just promising not to use it while someone else makes an illegal copy. This would invalidate copyright. That's why eBooks are treated more like software than books. Physically, they are.