Because animals are not machines. Breeding any animal takes it's toll on the animal including shortening lifespan.

I breed a litter when I have room for a new pup to run on. Other than that, puppy buyer wants do not factor into my plans. I am thankful for them when I have pups to sell, but my plans are for ME.

And just IMHO, producing puppies just to sell to pet buyers is not the kind of breeder I want to be.