Because they’re just not that different yet. Nobody seriously expects to see speciation occurring in dogs after a mere few thousand years.
Shall we engage in a little thought-experiment? Let’s say an event happens which causes reproductive incompatibility … the number of chromosomes changes, for example. According to your hypothesis, the breeders will observe that the dog has fertility troubles and think, oh, I better not give this dog any more chances to breed, because I don’t want to risk having a dog that can’t create lots of puppies? Whereas I think it’s more likely that they will have any number of other reactions, such as: “This dog is good at sheepherding, I think we should try again to get puppies;” “But my kids would like having puppies, let’s see if we can get some;” “tying up this dog is too much trouble because it barks a lot, if it has puppies with the neighbor’s dog, who cares;” or “I love Fido, I want more just like him!”
Remember that it is insufficient to your case for less fertile dogs to simply reproduce less successfully, because that applies equally to wild wolves and has nothing to do with artificial selection or domestication.
For the vast majority of doggy history, the model of puppy mills where the highest priority is on producing more dogs because people will pay for puppies is not relevant to how dogs actually reproduced. Dogs bred themselves as strays living in and around human settlements, or humans focused on a specific purpose they wanted dogs to accomplish. And even with modern puppy mills, they still usually stick within a breed.
Cross-breed fertility in dogs is a neutral trait, as far as I can see, neither selected for or against except for the general tendency of fertility to be selected for.
I will note that dogs are more fertile than wolves in a couple respects, in that wolves restrict their breeding seasonally and to when they have a stable pair-bond, to ensure that they have the resources to successfully raise young, while dogs need not factor in such things because humans feed them more reliably, so they can breed at any time of the year and with any partner they just met and still likely raise the litter successfully. But this doesn’t mean humans deliberately bred for this, either.