How many homeless people could one even very wealthy person help in San Fransisco or Silicon Valley? Even just building a homeless shelter, which I think are still uniformly pretty terrible places, is probably near-impossible; and not just because of the opposition from residents to them being built in their areas or neighborhoods, but also the regular, general opposition to any new development.
And just giving homeless people a better place to live is only one, tho extremely important, dimension along which homeless people can be helped.
And then of course one should expect side-effects to any help one would give, e.g. increasing 'demand' for help for homeless people. I'd imagine one reason why there's so many homeless people in San Fransisco is that it's already one of the best places to be homeless.