Because of exhaustion of rights working on first sale of right to _make_ derivative copyright works not being exhausted on sale.
In USA under Fair Use you have pretty liberal rights compared to most other copyright regimes AFAICT. In the UK we don't have rights, not even under Fair Dealing, to modify works, never mind sell modified works (without permission of the copyright holder); they made an exception to allow caching to make web browsing lawful!
I can't buy your book, edit out part I don't like and resell it as your book, and I can't sell it as my book without your permission. Not even one copy. That's reasonable to me (unless there was a [implied] license for modifications to be made, say). I should of course be allowed to modify the physical copy I purchased as that's ordinary use and copyright shouldn't interfere with that.
I would be very interested to learn I was wrong about that.
Hang on. Does "modified version" mean the same physical copy I bought and then modified, or a copy I made myself and then modified?
The way you worded that sounds like you're talking about both, but the former makes no sense. Why would it be a copyright violation to buy a book, white-out one sentence, and resell it? And elsjaako was definitely talking about the former.