I'm a serious composer, songwriter, musician, and sound designer, and this topic is definitely very interesting to me. Here's my take on this topic.
You guys are ignoring a very simple fact: musicians need to spend their money on their instruments, mic pre's, mic locker, outboard gear, studio maintenance, upgrading music-production DAW sequencers, sample libraries, plug-in instruments, audio effects, acoustic treatment...etc. Those costs in of themselves are already staggering. So musicians have to prioritize. I can tell you right now that no human being would refuse a very expensive and killer sounding audio system if it's given as a gift with no strings attached, including musicians. But with your own money, it's a different story. Musicians often have to put amazing sounding audio system lower on the priority list because it's not a necessity and they can make great sounding music without it. What's far more important is the instruments, mic's, outboard gear, software, room acoustic treatment...etc. Plenty of musicians are perfectly happy with a set of Mackie HR824's, because they really don't need anything better than that--it's pretty well extended at both ends, plays loud enough, is relatively more accurate compared to typical consumer stereo systems, and can reproduce the sounds they intended with their music-making gear fairly faithfully. There's no need for them to spend six or ten times of that to get that extra 15% in accuracy and resolution.
Even among musicians, there are different priorities. Some have ghetto audio reproduction gear but very expensive instruments like monster modular synth systems, or classical instruments, or vintage guitars...etc. Some actually do care very much about audio reproduction and buy esoteric DA/AD converters that are priced astronomically, and have speakers, amps, and whatever that rivals any audiophile. Some just use ghetto gear for everything, and only care about making music--as long as their finished tracks sound good enough where the average person think it sounds like the professional stuff they hear on radios, they're happy. Those who also take mixing and mastering seriously are usually the type who would have audio systems that rival the serious audiophiles, and many people in that camp aren't really musicians (at least not by trade)--they are audio engineers.
Also, the well-informed musicians have the uncanny ability to produce very high-quality sounding finished tracks using cheap gear--it is a necessary skill for modern day musicians when commercial recording studios are almost extinct now. It is also possible now because of the advancements in technology. I can put together an entire music-making setup for free and still create music that's high quality enough for commercial radio play. No, I'm not exaggerating--if you take a spin around kvraudio.com, you'll understand why this is possible. All you need is a typical computer--even a modest laptop will do. You don't need anything else--just freeware DAW sequencers, soundfonts, sample libraries, softsynths, audio effects...etc and do everything ITB (In The Box). Today's freeware and donationware are so high quality that even pros use them because they sound so damn good.
The truth is, if musicians still had money left to burn after they have purchased all the music-making gear they lust after, I can bet you that they'll target their sound system next, because they can afford it. But if all the music-making gear is already beyond their reach (such as that amazing sounding $5,000 mic pre, or that $10,000 Vienna Symphony Library), then it's totally understandable that they settle for average sounding studio monitors, audio interfaces, and headphones.
