Vitamin B12 is synthesized by microbes or through 72 step chemical reaction. The fermentation takes place in gut of herbivores by the microbes. Our ancestor were carnivores and relied on eating other animals for this chemical and out of that reliance we evolved the organs and enzymatic processes to a bare minimum that cannot satiate our needs of B12.
Besides what do you have to say about the idea about suffering of microbes? Do you think they suffer when they die?