Because it makes money and expands the visibility of the WWE brand.
Wrestling, because it's a self-sustaining product, is easy to air on TV. As long as it makes the network money, it makes them happy.
It would take the network canceling Smackdown for it to go away. Even when they have, they've simply changed networks. Still, it's been getting smaller and smaller with lower and lower stakes each time they've changed channels. I think it won't last another 10 years.