brake parts are expensive whereas rev thrust is virtually 'free'
Reverse thrust burns a bunch of fuel, so it's not as "free" as it used to be. However, especially in airplanes with older design steel brakes (747 Classic) instrad of carbon brakes (747-400, 777, etc), reverse thrust adds a significant safety and brake wear margin.
At least idle reverse is used by virtually all air carrier airplanes when fitted. Use of more than idle is up to local policies.