Do you think it just has to do with adding more layers and chances to mess up the signal? If not, why do you think it's like that? Why wouldn't they offer a bypass to go purer?
Volume control is a check-box that every playback software must have I guess. I think there might be some that believe it's a liability to have a volume defeat/bypass button because some users might think the volume is active and set low and then blow-up their speakers because it is actually bypassed. I'm sure that coloring the sound with volume DSP is not intentional, but I have heard it many times, more times than not.
At shows I have used DSP equalizers and crossovers and they always seem to color the sound, even when they are set flat. I avoid them now for that reason. I think the least coloration I have heard from an volume DSP is Amarra, Sonic Studio.
I believe that many manufacturers don't have resolving enough or low-noise systems that they can tell whether their DSP software is coloring the sound or not.
Steve N.
Empirical Audio