Because they're not getting enough well-paying gigs. Cars are more expensive? The cost of owning a home is higher? The average phone bill is higher? There are too many variables affecting disposable income to give you a definitive answer. You also need to define "working musician." Do you mean a musician who is playing pubs every weekend? Someone who plays every night in a resort? Actual recording artists? Session guys? Sidemen? I was a "working musician" in a Top-40 duo in the early '80s, and I averaged about $450 a week when we had steady bookings. I talked to a guy recently who plays in a duo, and he earns about the same amount per weekly gig that I did 37 years ago. It's not the cost of the guitar that's the issue. The problem is that those kind of low-level gigs just don't pay well enough.