Nobody concerned about rent price fixing thinks that we don't also need to build more housing here. This is just another part of the problem. Are you defending the practice?
Also - I don't think it actually affects prices. It's just that they've gotten good at seeking price equilibrium.
Last year, Seattle built 20% more houses than the highest of any of the previous ten years. (~13k units) And this year we're 11% above last year, so far.
What about that is not "actually encouraging more building"? I'd say your data might need revising.
Whether Seattle is building enough housing, or whether the region is doing enough to encourage housing development are separate claims.