Why do people (in general) leave Google for startups?
You can be an IC at Google... Or a director or VP at a startup. More scope and ownership can be more fun.
More risk = potential of greater returns
I left google to join fb. Some how i was not happy at work. I felt strange looking at people focussing more on design docs than quality of code. Design docs even for most obvious things. Also i joined on first week of 2017. Got cme followed by ee and did not get refreshers. Also i was down levelled by goog.
It's still a giant company and a lot of people will find it soul crushing. The gun slinger style of some startups may be more appealing to some.
I hear Google has smart people working on boring stuff, because there is nothing else better to work on there. Some people are fine with staying in that type of comfy job situation. Other people are more driven in life and want an actual challenge that'll make them learn and grow.
Because FB pays better and there’s Bootcamp
Boredom/Ego
They offer something different. Some or all of these may be true: less red tape, more ownership, wanting something different, more say in product or company strategy. May be accompanied by worse WLB, less $, less robust infrastructure. There’s trade offs to working anywhere