It's a lot of money but there's also a lot of people and a lot of costs in order to build.
Setting aside that you can have both vacant residential properties and homelessness, let's say the government decides to just build a bunch of units and force people to live in them:
Where do you put them?
For lots of the cities in CA you'd be looking at a million bucks or more for the lots of land alone in order to build like 4+ or 6+ multi-unit buildings. But let's be generous and call it a million (some property is already controlled by cities and such, after all).
So you're gonna have to settle for less than 17,000 buildings, since that's the land cost alone for 17B.
But ok, 115,000 unsheltered homeless in California, you can build denser. Too dense of just bottom-of-the-economic-ladder housing is going to lead to a lot of problems though, look at the history of housing projects. Let's do a 20 person per building one to try to get the costs down: 5750 buildings. Applying that same "million dollar lot" means we're at 5.75B for the land, now we need to construct 5750 buildings for 11.25B, about 2 million for construction per project... that's gonna be tough without getting more contractors into the market and driving down the costs of construction too in those cities. Cause otherwise having a bunch of new construction projects is gonna drive up the cost of construction, not down, unless you expand the supply.
And the more you try to push the density the more opposition you run into from both people who live and work nearby the sites and advocates wanting better housing. The latter is a problem IMO but it's not like getting rid of it would make the former go away immediately.
And you still need a large agency of operations around trying to find people in those units jobs so that you can get them out of the units before other people need them, etc.
Hell, why not just give about 100K to each of the unsheltered and spend the rest on relocation assistance to cheaper parts of the country? Sure, you could just try to bus people to cheaper areas without this, but that's gonna result in some deaths to do worse weather, fewer local resources and people to live off of, etc, so... it's unclear to me that even the cities in red states that love to make fun of CA homelessness would pull the trigger at that scale if they actually had to. There are a lot of problems they don't have to face because they don't have the scale of demand for land or the hospitable climate.