I can tell you the main reason. Liability for repairs.
because of the variables in pad construction and materials you very seldom get a nice smooth even wear surface. Usually if you measure from the center out you will find tapered wear and various dips and ridges where dirt, pad material, rotor hard spots cause uneven wear as well. If you install new pads onto that surface you may only have 10% of the actual friction surface contacting the rotor. Many say "well they will wear in" Yes they might wear in, but during that wear process the brakes are not performing the way they should. They don't transfer heat as well and the pads can easily glaze in locations of low contact. Turning the rotor so it is flat and even means much less chance of problems and the brakes work as they should as soon as they get installed.
Now consider the shop that installs the new brakes. They bring the vehicle in, brakes are worn. They replace the pads and DON'T turn the rotors. Vehicle owner picks it up and drives down the road. They are going a bit fast and do a panic stop, BUT because the new brakes are not seated and only 10-15% is actually working they end up in an accident. The tell the officer that "SHOP" just installed new brakes. care to guess who ends up in court due to the "pain and suffering" of the vehicle owners??? Insurance companies LOVE hearing that something was just done to a vehicle prior to the accident, they WILL tear into it and see if the fault can be handed off. In this case they will probably ask if it is industry standard procedure to install pads ONLY without turning or installing new rotors. BUT since they and the brake experts they call in know that it is a normal procedure they will want to know why "SHOP" didn't follow it.
I run into the same thing a lot. The shops have to do some things just to keep the insurance companies from wiping them out.
For instance many of the chain tire places and most of the box store bays will not install a tire other than what is listed as OEM size to the vehicle in question. This is due to getting sued for installing tires which the customer wanted on vehicles that then were either involved in accidents or other incidents.