It comes down to incentives. With the current policies in place, there isn't much incentive for incumbents / existing residents to allow more housing.
If you are a home owner or landlord, reduced supply results in asset appreciation.
If you are a renter with rent control, then building more housing in your neighborhood may not seem like much of a benefit either.
Policy makers are always looking for piecemeal solutions, e.g. let's give teachers a rental subsidy, tweak the % of affordable housing that must be build, etc.
Lawmakers really need to create incentives so incumbents have a reason to want more housing.