It's because WA allows for non-compete agreements for employees above a certain income level, whereas CA bans non-competes outright. So it's much more difficult to get top/high-ranking people to leave MS/AMZN since they might get sued by their employer.

Note that the threshold in WA is around $116K annual salary and I imagine that most senior tech employees would be above this level.

Source: https://foley.com/insights/publications/2023/01/non-competes...