Why is the use of reverse thrust not considered when calculating accelerate-stop distance on dry runway?
If your thrust reversers are operational and debris is not a concern, you're right for an in-service rejected takeoff (RTO): 


  There's nothing that prevents the use of reverse thrust on a dry
  runway for an aborted takeoff.


Most in-service RTOs utilize reverse thrust if available (see this video for a dry-runway example). 

However, in order to certify a transport-category airplane, one must show compliance with 14 CFR 25.109, paragraph (f), which precludes the use of thrust reversers on dry runways in calculating the accelerate-stop distance. The demonstration required by paragraph (i), therefore, cannot be conducted with reverse thrust on a dry runway. From a certification standpoint, the accelerate-stop calculation is made the way it is because that's what the regulations say. But why is the law written this way? See this video at about 1:12:


  The whole intent is to demonstrate [that] under [the] worst conditions you can safely stop the aircraft.


"Worst conditions" encompasses at least inoperative thrust reversers and worn brakes.