Neil Cybart, writing for his Above Avalon newsletter:
Given Apple's culture, the company is all about small groups and hiring people for a particular specialty. It has been reported over the years that there isn't much internal movement within Apple. It is not a coincidence that when looking at many of Apple's high-profile departures, the driving force often ends up being a desire for something different, chasing something that was not possible within Apple. Obviously, losing key people represents a risk for Apple. However, it is a risk for any company. There is no evidence to suggest Apple has an outsized employee retention problem compared to its peers.
I considered including retention in my biggest problems facing Apple in 2017 piece but ultimately decided against it for similar reasons. The only way to absolutely avoid losing brilliant, driven people is not to hire brilliant, driven people.
Apple does the opposite. They seek out and attract the kind of people who want to work on Project Purple (iPhone), Gizmo (Watch), or Titan (the rumored car project). They're the kinds of people who want to be part of the next big thing — of denting the next universe. Apple, by virtue of having brought several of those kinds of products to market and explored many more beyond, is one of the best opportunities to do just that.
But not the only place. Sometimes, the kind of person who wants to revolutionize phones will want to revolutionize something else, be it an app, an accessory, a service, or a combination of those things. If Apple has no interest in what they specifically want to do, they'll have to go somewhere else.
Apple also doesn't have a lot of room at the top. There aren't many people at the highest levels of the company and they don't turn over that often. When and if they do, and the position isn't filled by someone from the outside, it's still a single opportunity. Two or more people can't occupy the same space at the same time so, inevitably, some change spaces. And sometimes, after years at Apple, some people just want some time out, even if its just for the chance to build something of their own with all the tools they'd previously been making for others.
Once upon a time a bunch of people who worked on the original iPhone left to go work on the Palm Pre. That it didn't work out, so some moved on again and others came back to Apple. That pattern has repeated itself several times over the intervening years. Facebook was actively going after Apple talent for a while, giving people a lot of money but ultimately not much to do. Many have gone to startups over the years with some also coming back again. Now, Tesla.
People leave Apple. People also come back to Apple. Some do that multiple times. Others simply move around inside the company, looking for the projects that interest and excite them the most. Some have moved from iOS to watchOS or tvOS and some have moved back again or on to new projects. None of this should be the least bit surprising in a company Apple's size and with people this talented.
Brilliant, driven people are exactly the kinds of people Apple wants but are also exactly the kinds of people who can be hard to retain. That's why some who ran teams at Apple either leave or get poached to run entire organizations elsewhere.
There's already a senior vice president of software engineering at Apple and several absolutely top-flight vice presidents with wide-ranging skills beyond the bits, including interface and design, marketing and education, partnering and more. Project Titan's already got a software lead as well in Dan Dodge, former head of QNX — a real-time operating system at the heart of many existing infotainment systems and much, much more. In that environment, the biggest opportunities for movement, if movement is what's really desired, may well exist elsewhere.
That's not to say Apple can't pay more attention to their internal dynamics and do a better job keeping key people at the company whenever possible. They can and should always be doing more and better. It'll just never always be possible.