I believe they could have done this between the easy and normal difficulties because there’s not much of a difference between the two other than health regen and numbers for you and the enemies. But two reasons I see that they didn’t.

Madhouse difficulty changes so much it really wouldn’t be possible for you to swap difficulties mid playthrough, so maybe they just ignored it for the other ones. Or, the reason I think is most likely, RE7 has adaptive difficulty. So even if you’re on normal and struggling, things will get nerfed or you will get buffed after dying a few times.

If you had the preorder bonus(?) it unlocks Madhouse from the start which I guess could be a trap if you’re not ready for it and pick it.