I think MarkerInbound pretty much nailed it. Torque as an absolute number(or power/thrust) isn't a particularly relevant number to the driver. Does a pilot really care if the engine is putting a 1 million whatevers, or does he(she) cares about how much of what's available is being used? Hence percent makes the most sense. If the pilot is thinking they need more power and looks down and sees the gauge is already at 100%, then they know they better come up with another plan. OTOH, if the gauge says 50%, well then bring on the power
Same reason turbine engines display percent rpm, rather than rpm (ignoring for the moment that in most cases, redline is significantly higher than 100%
)