It’s a zoning issue but not in the way I think you’re saying. There is zero reason (aside from nimby-ism). There are houses in poor condition, why not allow homeowners to rebuild them into triplexes that are similar size to the houses near them (within a certain % greater), or convert their existing single family detached homes into duplexes? Or allow ADU’s on larger lots?

Most owners won’t do this, so it will be slow and gentle. Why not allow homeowners to allow their properties to work for their families?