There are a lot of myths about B-12. The most persistent one is that it comes from meat. It's actually made by bacteria. Vegetarians and vegans only lack it in modern society because of (a) antibiotic use, harming bacterial population, and (b) "clean" foods, which fails to reintroduce those bacteria, and which lack b12 themselves.

Not so surprisingly, the same is true for modern animals including farmed cows and pigs. I can't find a the source now (and apparently neither can wikipedia editors, cause they deleted it), but a very significant percentage of B12 production today (some say upwards of 80%) goes to animals. Some of it for their well-being, but most of it for the people eating them. A contender for the "least-efficient-way-to-supplement-vitamins", I think.

Also, many non-vegans and non-vegetarians suffer from B12 deficiency these days.