Plus those big companies recruit everywhere. I work at Microsoft and there are lots of UW grads of course, but there are lots of grads from WSU, Central Wash and Western Wash too. Some folks from Seattle U and SPU. And then people from all kinds of colleges and universities across the country. Yeah, some of the top schools are a bit overrepresented but a path from a smaller college to a top CS company is not unprecedented, especially if you work at other companies before coming here (which frankly is the trajectory for a lot of people). (And honestly, there are probably more WSU grads here than there are grads of some of the smaller elite schools. I’d imagine that companies like to recruit locals because they know that the locals will like the area and stay - they’re already here - and because relocation packages are expensive!)
And, as others have mentioned, there are hundreds of tech companies and thousands of other companies that require IT and tech expertise.