I think a lot of it has to do with driver designer’s failing to take the way our eyes see light into account.
As we have to double the amount of light to make a noticeable difference in what we perceive:
(Stealing one of Match’s output graphs from Emitter Test Results pt4: XML2, XP-E2, XP-G2, and Nichia219! (along with sinkpad tests))
Say, for instance, a designer wants a three-mode High-Mid-Low, so he builds a driver that steps in thirds from 3000mA to 2000mA to 1000mA. At 3000ma, the emitters is giving about 1k lumens. Step down to 2000mA, is using only 2/3 of the power, but is still putting out about 800 lumens. Your eyes would see this as being significantly close to the same output as the High mode. On the Low mode, the light would be operating closer to what your eyes would see as the Medium.
Based on this, a better possibility would be a mode group as:
High 3000mA, Mid 1200mA, Low 400mA