Because the outlay of breeding is a long way before the profit. Theres no guarantees of the number of pups thus what the income will be
I work in a rescue and we often get people asking to hand over pregnant dogs as they havent got the funds to support through pregnancy as are relying on pup money. Other than things like helping boxes etc Mum will need checks, possible medical care, pups will need microchips, vaccines and health checks well before you see any money from their sale.
Theres no guarantee your dog wont need things like an emergency c section out of hours etc or the pups wont need their own vet care. Just like humans theres no way of knowing if your dog will have a care free delivery and the costs add into the thousands quickly for emergency surgeries
Our latest mumma was handed in after she had a c-section, she then got an infection and needed post op care, she had 4 puppies, 1 died, 1 needed vet care. After all the vet Bill's, cost of micorchipping etc the owner was left in a significant amount of debt.
They told us that she was lucky she had 3, because if it had been 2 then they would have put number 3 to sleep as they wouldnt of had the money to pay the puppies vet Bill's.