Because the private market can't do it. When most cities in the US have 80%+ of its land zoned for single-family units, developers can not build anything other than expensive McMansions.
Don't forget parking minimums, rent control, environmental studies in excess of what is needed, and the endless town-halls full of people trying to veto change. All this just to build an apartment complex. You have to retain half the state bar to build anything around me and consequences are evident in the rent prices.
80% of Seattle is zoned for single family units. It's not exactly a suburban desert, but it could be much better. Unfortunately, even very limited legislation (HB 1782) allowing upzoning to duplexes/quadplexes ("missing middle" housing) within walking distance of public transit hubs failed earlier this year. NIMBYism abounds.
