1: The people are smarter. It's sort of cheating I suppose; better inputs make better outputs. Google doesn't have all the smartest people, but it's one of the few companies that get top-tier talent in a way that is meaningful. Everything else hinges on this.
2: Your manager (and their manager) cares a lot about you. That's a pretty broad statement and definitely not universal, but it's more often true than not. Managers are explicitly trained to focus on the needs of their employees, and the ones who don't are largely viewed as failures. No other success compensates for poor treatment of your reports. Again, there are plenty of bad managers, but they're widely seen as bad managers; justification like "demanding but fair" or "knows how to get results" or "evil genius" have little effect.
3: It's easy to move. There is no pay difference no matter what project you work on and internal mobility is encouraged and supported. People do what they want to do, and they tend to be extremely motivated as a result. It also helps remove bad managers: sinking ships get abandoned really, really fast. This is arguably the company's greatest weakness as well as it's hard to get people to work on something soul-consuming. But it's great for the employees and it's great for innovation.