New ones are round, they're shiny, they're clean, they're not rusted, and they're not bent. The flanges that hold the O-rings should also be straight and flat.
You can clean and shine up old ones, and you can usually see if they're not round or if they're bent. The flanges aren't too difficult to bend around and flatten.
And, of course, the new ones make money for the people who sell them. And those who work for them.
--DD