The output level is adjusted in the DSP and not by an output attenuator. Since the original design was seen as a "venue" (auditorium, etc.) unit originally, lower output levels were not seen a necessary or a desirable feature and would add cost to the user. The problem for sound mixers is that if the level were to be adjusted lower in the DSP, then the output D to A noise would become a problem.
The other thing is that we didn't realize was that some of the pro gear had a hole in their ability to handle input levels. It amazes me that line levels are so high and mic levels so low that they don't overlap.
As a band aid, we have a cable that has an attenuator at the mixer end to reduce ground loop noise and can be set for -20, -30 and -40 dB of attenuation.