On the engineering/product side of things, a lot of it boils down to the technical challenge of their interview process. Essentially, getting a job offer from those companies (and others of their ilk) show that you have very high capabilities. Their interview bar is so much higher than most other places, that you are proving yourself capable of the technical side of things.

That doesn't mean you have the soft skills or personality to succeed somewhere else, or even that your technical skills will matter to the new company's product, but it's attractive to the recruiters/hiring managers trying to fill roles.