OBDII requires full range misfire detection. Ford wants to be sure that most drivers never run out of gas which could possibly set a misfire code. When that happens, Ford pays for totally unnecessary warranty work.
Remember also that new Rangers come with 3/36000 Roadside Assistance which includes help if you run out of gas.
IMO, those are the 2 reasons why you get the low fuel warning and empty gauge reading so early - roughly 3.5 gallons before empty on a nominal 19.5 gallon tank.