Good luck getting anything built in Sim Nimby (you're going to need it). Support new development, affordable housing, more equitable living, etc.
Vacancy rates are very low in cities with high housing prices, and most vacancies in those cities are simply units changing hands between different residents.
As for investors buying up units, we know that they are not keeping them empty, because vacancy rates count investor owned units as well, yet have found the same very low vacancy rates. Why would investors want to lose money on an empty unit when they can simply rent out the unit instead?
Look up e.g. the Vacancy Report by the ACCE institute, which states: "With more than 36,000 unhoused residents, Los Angeles simultaneously has over 93,000 units sitting vacant, nearly half of which are withheld from the housing market."
Properties are often used to protect wealth from recession and inflation. Depending on local laws it might be considered too costly to have e.g. low-paying tenants as the value of a property is affected by occupants. A vacancy tax is often named as a way to force the property owners to have the property be of use to the public.