First, it's not that a HOUSE "should" be 3 x median income, it's the MORTGAGE. I'm not suggesting that everyone brings savings to the table, but it's a reality that as people increase their incomes/family size, they decide to buy a bigger and more expensive house. Commonly, they'll bring along the equity in the previous house.
EXACTLY!
That's been then Achilles heel of the whole median income/median price theory from the beginning: EQUITY.
Desirable areas like Seattle not only have higher wages, but also are drawing a large number of new residents from other high Equity markets, especially California.
Additionally, people who bought into the market early, often with what was then an affordable starter home below the median price, are bringing 100s of thousands of dollars in equity with them when they buy again.
For example, the previous owners of my house bought it for $160K in 97. They sold it for $430K in 2005 and bought a bigger house close by for $550. But I looked up their mortgage, and guess what? They're only paying a mortgage that is $214K! Very affordable for median income folks....and I'm pretty sure they're actually well above median income.
So you can see, the median income/median price theory is totally flawed. There's simply too many people in the market with large amounts of equity. That's why in a market like this you have to work your way up the equity ladder.