Why aren't semi's built to maintain speed limit up a grade under load?

asked the same thing other day and wanted to refine what I asked. I understand basic physics. A heavier load will require more energy for truck to pull, hense, lower speed. I am intereted why manufactures do not equip their trucks with enough power to make it up a grade and maintain posted speed limits. Would fuel cost not be worth it? I realize a 40ton truck is a lot to handle, but if a truck does this on a regualr basis, I wonder what impact increased speed would mean for fuel use, time, expense, tire load/speed, trailer stress, axle stress, etc. Maybe compare to mining trucks which always pull up hills an impact on cycle time ( I know different..)