I think in places like SF or Seattle it’s a combination of government regulation and zoning rules, and idle capital. But at the same time I don’t see why government itself can’t invest in building things like transportation infrastructure to support the increased density (assuming density is something we want - I’m not entirely convinced it is).
Over the last few decades the default state appears to have been “build up” rather than “build up where needed, and out where you can”. Cities from previous iterations of civilization appear to have done the latter, while we mostly just do the former here in the states (over recent decades). LA could maybe be considered an exception, however it’s zoning rules are draconian and density seems to be the way forward less some major overhaul.
The problem is that these things take time, mostly because this isn’t China and Seattle doesn’t have hundreds of billions in Treasuries to do what it wants, nor is it willing to conspire using shady financial vehicles like Chinese local governments.