Most of the interstate system was built before the 55MPH national speedlimit. The open roads were designed for 70MPH or better, and that was using 1960s era equipment. Some urban interstates may be designed for 55MPH, but in general that is not the case.

I think we have three limitations to higher speed limits in the USA - the low standards for driver training, the low standards of maintenance of vehicles, and the vehicle choice. The first speaks for itself. For the second, think of underinflated tires. On the last, SUVs are not made for high speeds - 75MPH is plenty, given the generally crude suspensions, weak brakes, and rollover proclivities.