1. People don't know that they can do that.
2. People don't have the patience, energy and financial resources to sue everyone copying things from their personal websites.
So yes, you could put something like this on your website. But let's be honest, this isn't good for the "free" and "open" internet that we all want. 1. All airlines in the world.
2. All cars in the world.
3. All factories in the world.
4. All rockets in the world.
Right now, I think that 1, 2 and 3 combined have much deeper consequences than 4.