It's in the name. Copyright. As in, the right to make a copy.

It's perfectly legal to sell a digital good as long as you don't retain it as well.
It's illegal to make a copy of a book and then sell that copy.

From probably the most biased source possible: https://copyrightalliance.org/education/copyright-law-explained/limitations-on-a-copyright-owners-rights/first-sale-exceptions-copyright/

As they point out, most digital works are licensed, not sold, so there are terms and conditions associated with how you can use them.

So it's perfectly consistent, just grossly out of date for it's intended purpose of "make sure writers can make money selling their books without worrying that getting copies made will be pointless because someone else will undercut them and leave them with 1000 prepaid copies of their book that everyone bought cheaper".

We should have a system that preserves that original intent of "creators get compensated", without it turning into our culture gets owned by some random company for more than a lifetime.