Because new housing doesn't magically appear around existing tenants and their furniture. Existing tenants get told to leave, then there is construction for x m

Because new housing doesn't magically appear around existing tenants and their furniture. Existing tenants get told to leave, then there is construction for x months, then a new building is there with more units available, and (especially if the old building was under rent control) each new unit costs more than each old unit in the building that used to be there. And its possible none of the tenants who used to live in that building can afford to live in the replacement units - tada, displacement by adding more apartments.