The cheap (exspensive/ junk) metal used to make rotors nowdays is harder to get a quality cut on in my exsperiece. They dont seem to stay true as well. That, along with the fact that rotors rust and deteriorate on surfaces that cant be turned make it easier to just buy new.
You can turn a rotor and still end up with a shimny or viberation because the cooling fins can rust away and throw the rotor out of balence. Twenty or thirty years ago you would never notice a unbalenced rotor because cars were not that smooth. Now cars are so refined you notice everything.
For most shops it doesnt make sence to turn when they can just slap on new ones and charge you the same $100 per hr. Plus mark up the cost of parts.