Because sound isn't the same as electricity.
Basically, our ears hear logarithmically, they don't hear things linearly. For instance, it takes 10 times the power to get twice the volume. So a 100 watt amp is twice as loud as a 10 watt amp, not 10 times louder. If it were linear, it would be 10 times louder.
So, when we build pedals, we use A value pots for things like Gain and Master volume because that's how our ears work.
Jacob