The density of investors and other founders/early employees in Silicon Valley can't be beat, and I often found myself learning inadvertently - dinner table conversations would be about how people raised their round, the friend of a friend you meet while camping turns out to be a partner at a fund you're trying to pitch. In more cases than are logical or fair, people simply invest in their friends - which means if you make friends there you're increasing the chance you'll be able to raise money easily.
That being said, the cost of living is absurd and I would never hire an engineer in Silicon Valley unless I was building something at the outer limits of frontier tech (and even then I'd scour the globe for qualified engineers elsewhere). And I don't think you need to be there forever. If you build up a solid network you can move away and still reap a lot of the benefits from afar / through occasional visits.