Classical and Jazz musicians are usually part of their Musician's Union local. Therefore, they are paid scale. I don't know what that is these days. They also pay dues. Again, I don't know how much that is these days, but if you aren't playing paying gigs all the time, you just aren't going to get paid a lot.
It's not like a 9-to-5.
On the other hand, a very small number of people are being marketed by Burbank (the city is generally associated with record labels (CD's now, of course.) You may think they're making a lot of money, too, but they aren't: they're considered milche cows by their labels, forced to tour to "raise awareness" for their albums, and then charged by the label for everything: transportation, food, lodging, techs, roadies, equipment, trucks... and by the end of the tour, they get their measly part of the box-office take...which is not much.
In short, musicians don't make much at all, and your second grade teacher (who only actually works for 40 weeks a year) makes more than they do...lots more.
Why is it like this? Well, ask yourself. When is the last time you paid to see a string quartet? A jazz ensemble? How many people were in the house, if you did go? Would you pay $100 for a ticket to an orchestra concert in a hall that holds 500 people? (After all, there's on the order of 60 people in the orchestra, not counting stage manager, librarian, empresario, etc: those folk get paid too, but don't make music!) So figure: 500 people (if they're lucky) come and pay 15 bucks a ticket. 60 people play music for 2 hours. This is all the money-making they've been able to do this week, because they have to practice 6-10 hours a day to 'keep up their chops'. Do the math. If the musicians could split the take only among themselves, that's $125 each. How long could you run a household, perhaps with children, on $125/week?
So the solution to poor musicians is for people to pay, willingly, to attend concerts. And what's the first thing people cut out of the budget when the economy gets weak? Entertainment! It's so much cheaper to stay home and listen to recordings.
This is a constant problem, and a vicious cycle: if you can't afford to rent the hall and advertise and pay musicians a living wage, you can't put on concerts. If you do, you have to charge enough per seat to pay the bills...only you also have to charge what the market will bare. If you don't have concerts, no one will care when you do: they'll be conditioned to ignore it, as we all have been for a half-century.
This is why so many symphonies are closing up shop and pushing their musicians out the door.
It wasn't actually much better 300 years ago, but at least, then, there wasn't an easy replacement for live performance. With radio, TV, recordings, and pirated MP3's floating around the Internet for anyone to take.... It's amazing we have any musicians alive at all!