With private sales, bragging rights sell, so Ford wants to show the biggest numbers reasonably possible. Realistically, how many people drop $70-90k on a truck and go around with it floored at 5,500rpm with a heavy load.
For fleet sales, detuning it a bit for more long term durability is a selling point. If they really need 400+hp, they are likely moving up to something heavier duty with a diesel. Also, keep in mind, the driver has no skin in the game, so it's desirable to have the engine protect itself from an idiot driver.
MH's happen to use a commercial chassis, so they aren't going to uprate them just for that use.