/shrug
I’m going to say it’s RNG. Used to be I’d find them in piles while looting houses. My last run, with item generation nearly doubled, I found one stack of 20 multivitamin over the course of an entire in-game year. I was actually paying a lot of attention to this because it was the first time I was playing with regular nutrition. I’ve found hundreds of calcium supplements in that same period of time and many other medical items.
Either someone turned down the spawns (seems really unlikely) or you and I are both just unlucky.
EDIT: Of course, I say all that and then start playing that save. Found a stack of 20 multivitamins in a grocery store, and another 20 stack in a house right next to it. Hooray . . .