It may not be the rotors. What problem makes you replace the rotors? Warped? Worn too thin? Grooved? Is the miles between changes the same every time? Do the pads get replaced every time?
Here are some possibilities:
Warped rotors may be caused by over-torqued or unevenly-torqued lug nuts, some cars are very sensitive to this.
Grooved or unevenly-worn rotors could be cause by worn-out pads or stuck calipers
Mechanics can over-state the need to replace the rotors
Riding the brake can wear out pads and rotors prematurely.
Low-quality replacement rotors can wear out prematurely - did you use name brand?
More information would help get you a better answer.