Because it has endless strings, hoops, paperwork, and relinquishing freedom attached to it that no one in their right mind would go through or agree with, let alone people who are homeless and barely have the capacity to wake up in the morning.
For some yes, but I'm specifically talking about the cohort who choose that lifestyle and wouldn't take housing no matter what. They prefer to live in a tent in the hillside, with not a shred of responsibility or authority telling them what to do. Many use substances. Our modern society has largely given them the Ok to live this lifestyle with minimal friction, and so they will continue to live it because it's what they desire to do.
