Why? If California is your norm, keep in mind that California has 20% of the U.S.'s homeless population (California, Texas, and Florida have 58% of the under-18 homeless population). It's really not a universal problem.
I can understand the reasoning, but in fact there are many homeless in northern cities. New York, for example, has a large number. Perhaps migration isn't common among the homeless.
That number is the result of the massive, largely volunteer effort led by HUD to count the homeless that is done every January. There are other numbers, but they tend to involve a lot more guesstimating and a lot less data.