I think it's because homeless shelters can only take so many people. ANd it doesn't provide you with ALL the things you need. And, it's for only a period of time you can stay there. So many homeless people choose to stay on the street then go to homeless shelters.
Not all homeless people are allowed to use homeless shelters. Many of them are using drugs, and need to pass a urine sample before they can stay. A lot of them have been apart of illegal behavior, and before staying at a shelter, need to have an interview before staying there. If they do not meet the standards they will either not be allowed to stay, or will be turned in. Something that might encourage someone to stay, is they have a lot to offer, such as showers, food, a bed, and sometimes even support finding jobs and so on
