I have a lot of sympathy for your argument that this problem is making it worse for everyone. I live in SOMA in San Francisco and regularly encounter many of the issues you describe. It makes the quality of living worse for the whole neighborhood, and I do think it is in part caused by very lax rules about what is acceptable in public spaces.
That said, I think you under-estimate how hard it is for someone to move once they are experiencing homelessness. I doubt the majority of people in this situation are doing to to pursue a music career...
This isn't about entitlement.
If your only option is to live in a van, your day to day existence is going to be very stress-full trying to scratch out enough income for your basic needs, and parking tickets, impound, etc could put you into an even worse spot that you can't recover from.
The idea that you should just up and move to a cheaper place to live sounds reasonable on the surface. But imagine all you have is that Van, you don't have enough money for any major repairs, let alone hundreds of dollars in gas to get to some cheaper place. You don't have a network there, and you don't know your way around, so what are your job prospects?
The reality is that when you're living this close to the edge, you're only thinking day to day, because you're always one problem away from ending up in an even worse situation. Packing up and going some place cheaper is far from a viable option.
Finally, most people who are now homeless in an area were previously housed in that area, so they probably know people there. They may also have some kind of public benefit, and have some clue about how to navigate that system. Packing up and leaving, especially to another state, starts them back at zero.