Because that’s not what happens - people will poach the rare ones and sell the parts for money, and for endangered species that causes bigger issues. There’s no resource that checks you got a roadkill, it’s just outright illegal. So it’s supposed to protect them from poacher - similarly you can’t sell thier bones, skin, shells or other parts online either.
As usual, low integrity people is why we can’t have nice things.
You can buy animal bones online, ebay… Etsy etc. I got a raccoon skull for a specific vessel on Etsy. Raccoons are not endangered and thier bones are legal. My vessel: Artists! Painting skull! - #7 by Mulberry