My guess is laziness. It's easier to inflate the tires to what's stamped on them versus looking around the car to find the appropriate value. You may get slightly better fuel economy by inflating the tires to their maximum safe inflation, but it could sacrifice tire wear and vehicle handling.
The manufacturer recommended inflation is based on the vehicle weight distribution as well as, to some extent, alignment specifications. Very few tires are designed specifically for an individual vehicle which is why what's on the sidewall typically differs from what's on the placard.
If you happen to know what the vehicle weight distribution is front to rear and do the math you'll likely find that the recommended front and rear inflation values in psi (pounds per square inch) calculate out such that approximately the same contact patch for the front tires is similar to the contact patch for the rear tires. It might be slightly biased toward the front to account for cornering and braking load differences since the front tires carry more of each of those loads.
-Rod