B12 deficiency is almost unheard of outside fairly uncommon genetic disorders or areas of extreme famine (or persons with extremely compromised diets like alcoholics, Bulimia nervosa, Anorexia nervosa, etc.) and until the last few decades people simply didn't live long enough to start to have B12 deficiencies caused by age.
The body requires so little B12, and the source of it is so readily available, and you don't even need to eat foods rich in it every day, that even in absolutely undeveloped countries, people are able to readily find sources of B12.
As omnivores we evolved in a way that we can't synthesize B12 internally so we must eat food sources that contain it.
Thanks to the miracles of modern food science even people who purposefully avoid the one source of B12 we evolved over millions of years to eat can find foods artificially enriched with it (take that all natural diet!)
> Analyzing data on more than 10,000 patients, University of Kansas researchers found that 70 percent were deficient in vitamin D and they were at significantly higher risk for a variety of heart diseases.