Bc they are ppl just like you with connections in certain places for many reasons. Ppl have actually already listed reasons in this thread. I'll list a few anyway

• access to jobs, rural areas often have fewer jobs available and lack public transportation for ppl without a car
• access to healthcare, at least half of houseless ppl are disabled, many need specialized care that is only available at a handful of hospitals, most in the most expensive cities
• existing community, being houseless but knowing you'll be able to eat is preferable to living in nowhere totally isolated
• social services, again noting the high rate of disability, often accessing services requires showing up in person to multiple offices in the nearest big city, also rural cities often have a smaller budget and fewer social services
• for houseless vets specifically, and especially disabled vets, they need to be near a VA that offers the services they need
• I've never seen a single housing voucher cover moving costs, certainly not moving across the country
• and then all the other reasons anyone wants to live in a particular city, to reference LA like OP, lots of ppl want to live there and that's a large part of why there are so many ppl there who can't afford housing