"Widespread" doesn't necessarily mean that sufficient quantities make it into the diet of humans. Some herbivores harbor a reserve of the bacteria in their dig

"Widespread" doesn't necessarily mean that sufficient quantities make it into the diet of humans. Some herbivores harbor a reserve of the bacteria in their digestive tract. Perhaps that's because eating widespread bacteria doesn't give you enough; the B12 must be concentrated somewhere. Also, humans spend very little time eating. It's much more believable that a cow (which eats all day) somehow ingests enough B12 from widespread bacteria than a human which eats for a few brief periods.