Buried deep in many manuals and publications is the recommendation for increased pressure for more demanding conditions -- speed, load, road condition, tire condition, etc. Tires work because of the air, and they work better with more of it than the usual default automaker recommendation.
As far as I can tell, the default recommendation by the automaker is made by the marketing department to sell cars with a softer ride, limited by the legal department (who consults the engineers for the bare minimum) to avoid blowouts. Remember the Ford/Firestone debacle? They either forgot to consult with the legal department, or the lawyers forgot to ask the engineers...