I am inclined to agree with the OP, mostly. I believe more-varied kinds of stories have emerged in my 5E campaigns than would in any single PbtA or FitD game. The familiarity and penetration aspects @TwoSix mentioned are real, of course, but those seem more applicable to finding tables or players. The fact I'm extremely comfortable running 5E and don't think I'd be at all comfortable running anything FitD or PbtA seems relevant here, too.