Why are heart rate zones only shown on limited workout types?

I know that heart rate zones only show for cardio workouts on Apple Watch, but why doesn’t Apple consider treadmill or outdoor walks as cardio? An outdoor walk at a slow pace but steep incline could still push your heart rate into higher zones. Being as the function is already there to show the zone, based on Health personal stats (age) and current heart rate, it should be easy to show the zone for any type of workout.