Because you can drive such a truck https://preview.redd.it/jxpnmzv586e81.png?width=1024&auto=we... if you do not need to transport a family and a sedan and those two will weigh less and have better combined gas mileage than american larger truck. If you relax the assumption that you always transport both a family and a truck load you allow for more powerful trucks and still come out ahead.

In addition you do not need to waste gas if either capability goes unused and the mileage of transporting people won't depreciate the more expensive truck faster.