I think the item you're missing is that because they are fuel-inefficient and quite old, the planes are acquired by Fedex/UPS/etc for a very low price after hav

I think the item you're missing is that because they are fuel-inefficient and quite old, the planes are acquired by Fedex/UPS/etc for a very low price after having been used by passenger airlines. If you can buy something for very cheap, and only need to use it infrequently, it isn't a huge problem if it's inefficient for that use a few hours per day.