How can something being fatty be bad if saturated fats are what the body uses to make replacement cells for your skin and your brain? I mean, yeah, your body will turn other things into fat to make those cells but it's been shown that it'll go after fat first as choices of building blocks.

Where do you think the B12 in the dirt comes from? Those bacteria are produced and live in digestive tracts. Ruminants reabsorb that B12 into their meat by re-eating previously digested food, humans don't get that option unless they're eating ruminant meat or their own poop (or the crap of others, though I think that goes against the 'no animal byproducts' part of veganism. Not sure where that line is drawn, or if it's a moral imperative or not.).