Yah. They're in high demand right now because people can't do this. xD The supply remains low because the person who hatched the pet will often name it after a little while; they want to be able to hatch more Creatu, and keeping it unnamed keeps them from doing that.
If you could do this, there would be unnamed Creatu all over the place. They wouldn't be worth as much anymore.
However, you can let a pet die and become unnamed again. You just.. can't do anything with it while it's dead. So in a way, this isn't much better than keeping it in your hatchery, since you can't train or anything, and some people will even refuse to buy 'mistreated' pets.