It honestly might not be. There is too much expectation on Half Life 3 that it would not live up to the dreams no matter how good it was. Invariably this would hurt Valve’s image with future customers if they felt dissatisfied with a product.
Besides, DoTA 2, Steam, CS:GO, and TF2 bring them boat loads of money without actually having to do much. They aren’t a game developer so much as a service platform now (and I include all the hats & gun case sales as really just making it all about service).