Another thing to add...
Rotor tech hasn't really evolved a lot, but brake pads HAVE... whereas it was common to change pads frequently, and they were 'sacrificial', now they last 50,000 to 100,000 miles, which wears rotors more. Another reason to swap out both is cost... Rotors are cheap, and it's a quick swap when doing pads, vs adding hours of labor... On my Audi i believe the OEM rotors were only around $30 a pop