There are dozens of vacant homes in US for each homeless person. The national vacancy rate is something like 10%.
Furthermore, people aren't exactly keen on letting homeless people live in their property for free. Hotels in my city experimented with this: they got small checks for housing homeless people during COVID, when lockdown massacred the hospitality market. It was a complete disaster: the rooms were trashed and they lost more money than they received from the government.
https://arstechnica.com/tech-policy/2022/10/company-that-mak...