Seattleite here. One of the reasons it's hard to "just build more" here is that the city of Seattle is bounded on two sides by water (Lake Washington to the east and Puget Sound to the west). This means that there are far fewer square miles available for "bedroom communities" outside the city; this, in turn, has caused massive pressure on rents and housing in those communities that are still driving distance from the city (modulo Seattle's awful traffic, which is itself driven in part by geography and is among the 10 worst in the nation[0]). There's just not as much land to build on--and most of it is already built on.
[0] http://www.seattletimes.com/seattle-news/transportation/seat...