I think it's more like there's more surface area to forget when you have humans handling so many concerns, and it's not likely the part that's changed the most

I think it's more like there's more surface area to forget when you have humans handling so many concerns, and it's not likely the part that's changed the most so it's a likely candidate for being "pushed out of the buffer" (of the human). In a more typical model, backend devs focus more on security, while not needing to know the frontend, and vice versa.