Quote from NoDoji:
Bacteria in soil and water manufacture B12. Our own bodies also manufacture B12 and it's stored for a long time because we only need a tiny amount of it. This is why even a pure vegan who washes his/her vegetables thoroughly wouldn't need supplementation for several years
Animals get B12 by eating soil containing the bacteria that produce B12, so if you eat the dirt on your organic veggies, you don't need B12 supplementation.
Also most cereals are fortified with B12 as well, so the majority of pure vegetarians who either buy dirt-sterile conventional produce or who wash their produce before eating it, are getting the B12 they need.
A better question would be why do Atkins, Mercola, Weil, etc. promote so many supplements for those following their recommended ways of eating? If meat and dairy products in the diet are the best source for for well-rounded nutrition and they're all promoting fresh produce as well, then why is any supplementation at all needed?