My doctor explained it like so: B12 is produced by bacteria that hang out in dirt. Us humans have largely stopped chewing on dirt directly, so for a while, it was advantageous to have that middleman of animals eating plants+dirt and then eating those animals or what comes out of them.

But now it's becoming rarer for farm animals to chew on dirt, too, because they might never see a meadow and instead just get fed soy or corn or such, to bulk them up as quickly as possible.

And so, yeah, she actually recommends B12 supplements to everyone. B12 is just consistently low in blood tests and well, she's also obviously not going to recommend chewing dirt when supplements are cheap and easy to get.