I don't know if it's just about the money ...
'Back in the day' when I was in high school just starting out, the guys I was with bought our 'band jackets' at a real cheapo place. wore a simple white shirt and thin black tie and black slacks - nothing expensive but we looked good -

I think it's more in line with what Diki said about the "slacker" mentality ... We went to see Spyro Gyra at a nice jazz club in Boston a year or two ago ... three of the members were wearing faded jeans and faded polo shirts - including the leader Jay Beckenstein ...

I think it also reflects our society's lack of RESPECT for anything or anyone ...

I've been to a number of outdoor concerts this summer, and the dress on a lot of the musicians is what I would call "sloppy" ...
I don't say they have to be 'formal'. especially playing an outdoor concert. but at least look 'pressed' ...

When I played that wedding in North Carolina. I wore a tux ... the sax player that was hired to work with me wore a 'sport jacket', slacks, shirt and tie ... On a break we were discussing musicians dress, and even the grandfather of the bride, who is also a working musician. said that wearing a tux looks "dated" ...
I guess for gigs like weddings and such in nice catering halls or country clubs, I am going to look 'dated' ...
t.