Like others have said, it takes time for B12 to be produced in significant quantities, and by that time it has passed the small intestine where it can be absorbed.

I'm not sure, but dogs have significantly shorter (looking quickly, 1/4 the length?) digestive tracts than we do. Less surface to absorb things.

probably produced too late in the process...

Rabbits for example re-eat every piece of faeces because the digestion of grasses take so long it takes two passes to get the energy out...