There are plenty of reasons why a homeless person might actually prefer not to go to a shelter--contagious diseases, tainted food, sex offenders, abusive practices by shelter staff, and better alternatives, like sleeping in one's own car. Despite all these, shelters remain continuously full, and people are turned away every night of the year. That's because there are so many homeless--more than 12,000 in Denver alone.
Homelessness is one of the few "growth trends" in our present economy. And it is, indeed, a purely economic phenomenon. Nobody chooses to be homeless. Kerouac was talking about low-budget tourism--a very different thing. This is not tourism, not a vacation, not camping. Let's be clear about that.
The three leading causes of homelessness are job loss, the high cost of housing, and break-up of a family. Mental illness and substance abuse do not cause homelessness. There are plenty of people who have these problems and yet have a roof over their heads. They have housing for the same reason you do. Either they have the money to pay for it, or someone who has the money is willing to provide a home for them. Those are the only reasons why anyone has a home.