only defense, interstate commerce, international relations, and the like are national problems.
the state is the canonical unit for a problem like homelessness, and each state is free to delegate to local jurisdictions or to coordinate with other states on solutions.
by the way, san francisco has about 8000 homeless folks, and LA has ~55,000 (~40% of the 130,000 homeless in the state). if we wanted to build new housing for all of these folks, we'd need about $50B (state budget is about $200B/year). LA, for instance, spent $0.6B on homelessness last year, or about 3% of what is needed just in LA.
we either continuously pay for homelessness indirectly (and ineffectively), through police, crime, social services, drugs, non-profits, governmental programs, emergency services, etc. or actually tackle the problem directly by raising and allocating that $50B.
(obviously talking about first-order effects here. this is not an in-depth policy paper.)