you cannot lend to as many people as you'd like
How often do you actually do that?
you cannot keep a personal backup copy
But someone else does that for you. And with Calibre, you can do it yourself.
But you usually get it for less than the cost of the physical book, so you already took the price break of selling it later, up front.
you cannot read it on anything other than a Kindle.
Except a phone or a tablet or a computer.
And here are some things you can do with a Kindle eBook that you can't do with a physical book:
Own 4700 of them without taking up any shelf space or having to move heavy boxes.
Take 4700 of them with you on vacation.
Carry 4700 of them with you in your backpack.
Have one delivered to you a minute after deciding you want it.
Gain immediate access to a book you forgot to bring with you. Even if you didn't bring your Kindle.
Have it delivered without a gas-guzzling truck pulling up to your door and a package to dispose of.
Do a word search.
Make highlights and have electronic access to the parts you've highlighted.
I, too, would like them to be cheaper, but they aren't directly comparable to physical books. Things change. Progress is made.