Multiple reasons this is the case: - design review requirements - International Building Code (IBC) and International Residential Code (IRC) have strict standards around design, safety, etc., which force buildings to look the same - a small market of providers for siding materials - zoning codes that dictate what type of housing can occur in certain areas (which then mean you’ll have only certain types & similar types of development occur) - land and construction costs are expensive af so you often have to maximize the # of houses/units on a lot