Often, that isn't possible due to zoning in these 'unrestricted' cities. Zoning tends to be really restrictive, up to the point of placing tight limits on the density of dwellings.
It's slow because you need to wait for many individuals in a market to act. They'll usually need to act in cooperation, piecing together smaller parcels. It's expensive because you are destroying housing to make housing.
Also, management of public infrastructure (even schools) is not flexible, adapting to local needs in realtime. A planner/zoner may look at an area, see that the infrastructure is insufficient and shut down population increases because they have no way to pay for more infrastructure.