The vast majority of homeless people are not visible, and they are not the stereotype of the drunken incoherent bum sleeping under a newspaper on a park bench like the guy in Back to the Future.

It's startlingly easy to become homeless simply by having a minor upset in your income, which can get you evicted quickly if you're renting and especially so if you live in an area which has weak or nonexistent tenant protections. Lots of homeless people were doing just fine or at least close to okay before something happened. They got injured and thus lost their job. A spouse divorced them and took most of the income with them. Their house burned down but they didn't have enough insurance to cover it. They had to escape from an abusive domestic partner. Etc.

These are just ordinary people who had their home pulled out from under them for some reason. Now they're temporarily living on a friend's couch, or in their car, or in a motel room, or whatever. But the barrier for entry for obtaining housing is so damn high in many places that it's impossible for them to work up the capital to make it over that hump and either make rent plus a security deposit, or magically cough up the down payment on a mortgage.

Many of these people probably already owned a car before whatever it was happened to them and thus they still do. Even if they're still paying off the loan on that car, that monthly payment is almost guaranteed to be less than rent or a mortgage.