Because the engineers who design these things aren't doing it on a case by case basis, they have to settle for general-use specs. We make all sorts of modifications to how the factory engineers - and accountants - built our trucks, based on our intended use. Mine is rather heavily modified, and does the job I want it to do MUCH better.
You can run max pressure all the time if your option is to trade a little harder ride for a little better MPG, or go softer and make the opposite trade.
Having said that, 60-65 is probably happy medium and the best way to fine tune from there is to try different pressures and see where you like it best.
