Vet bills can easily run into thousands, puppies need lots of time, food, medicines etc etc that all costs. They may live at home for extended periods before finding the right home... etc etc
And if anything goes wrong then the costs can be even higher - losing a beloved girl during whelping is a big and very real risk and good breeders only undertake breeding very seriously indeed.
Accidental breedings can and do happen - I can personally vouch for that! And sometimes all the intervention in the world after the event doesn't stop nature's course. But deliberate cross breeding is a different matter and I can't think of a single example where I would think it was ok.