These cities are only dense in the American sense. Even Manhattan looks unremarkable next to a properly dense city like Sao Paulo. Los Angeles is probably the best city suited for becoming really dense. The basin is sprawling, it can fit over 20 manhattan islands, but most of it is low slung single family homes that could one day be upzoned into towers. Lots of areas near downtown are just industrial/warehouses too and could be converted into housing in the future.

It has to happen eventually. As long as there is economic activity, there will be jobs hiring the best talent from around the world, spurs more economic activity, etc. That's just not going to change, demand will continue, so cities really need to get rid of zoning for height so supply could be built near the job centers. The alternative is sprawl, which means people need to be transported, consuming resources, etc. Infrastructure is also all the more difficult to build out over sprawl. In particular for California, adding to the sprawl could also mean building into wildfire prone areas.