There's a fair amount of buildable land in Seattle. The trouble is the building industry only wants to build flats for techies who can afford million dollar condos, not housing people with ordinary jobs could afford.
Largely because of restrictions put on the developers/builders and the cost of land. It isn't economical to build "normal" housing in cities. They have to sell/rent them for a lot more to make their money back, and in order to sell them for those higher prices, they have to be desirable to people willing to pay that much.
I mean, I would love to see apartments with formica, linoleum, and carpet, but that won't happen, especially downtown, unless they can get 1000s of units in a building, which isn't feasible under current building codes. Providing bonuses for providing more affordable apartments and condos is the only real way to get working-class housing downtown, but the politicians are so incapable of thinking that way, it won't happen.