Do you also think that Ford and GM should get more money when someone sells a used car, or that builders should get more money every time a house gets sold?
The difference is that with a house and a car, you pay for the physical object. With a book, you pay for the ideas contained therein - a subtle but (IMO) important difference.
Which isn't to say used-book sales should be illegal, but that if you can afford it, why not support the person behind the ideas you're profiting from?
"With a book, you pay for the ideas contained therein - a subtle but (IMO) important difference."
Not according to the law, you aren't.
Also, there are plenty of ideas involved in building cars and houses. They're not just random piles of wood, metal, and plastic, any more than a book is just ink smeared on paper.
Which isn't to say used-book sales should be illegal, but that if you can afford it, why not support the person behind the ideas you're profiting from?