Factory farming is a consequence of a post-industrial economy where 95% of the population isn't directly involved in farming. Few people would want to reset the clock back to where most are attached to the land with limited options. The only reliable source of B12 before the modern era was to consume some animal derived products. Other basic nutrients are hard to attain through plants alone. It is necessary for us to engage in animal husbandry in the absence of technological interventions that we never evolved to depend on.