I don't see the value in lamenting the old days of a few machines where you could actually name them as Middle Earth characters, install individually, log in to one single machine to debug a site issue. The problems were smaller and individual server capacity in respect to demand was in meaningful fractions. Now the demand is so high and set of functions these big companies need to offer are so large, it's unrealistic to expect solutions that doesn't require distributed computing. It comes with "necessary evils", like but not limited to configuration management--i.e. ability to push configuration, near real time, without redeploying and restarting--, and service discovery--i.e. turning logical service names to a set of actual network and transport layer addresses, optionally with RPC protocol specifics. I refer to them as necessary evils because the logical system image of these are in fact single points of failures. Isn't it paradoxical? Not really. We then work on making these systems more resilient to the very nature of distributed systems, machine errors. Then again, we're intentionally building very powerful tools that can also enable us to take everything down with very little effort because they're all mighty powerful. Like the SPoF line above, isn't it paradoxical? Not really :) We then work on making these more resilient to human errors. We work on better developer/operator experience. Think about automated canarying of configuration, availability aware service discovery systems, simulating impact before committing these real time changes, etc. It's a lot of work and absolutely not a "solved problem" in a way single solution will work for any scale operation. We may be great at building sharp tools but we still suck at ergonomics. When I was at Twitter, a common knee-jerk comment at HN was "WTF? Why do they need 3000 engineers. I wrote a Twitter clone over the weekend". A sizable chunk of that many people work on tooling. It's hard.
You're pondering if hiring practices and turnover might be related? The answer is an absolute yes. On the other hand, these are the realities of life in large tech companies. Hiring practices change over years because there's a limited supply of of candidates experienced in such large reliability operations and industry doesn't mint many of them either. We hire people from all backgrounds and work hard on turning them to SREs or PEs. It's great for the much needed diversity (race, gender, background, everything) and I'm certain the results will be terrific but we need many more years of progress to declare success and pose in front of a mission accomplished banner on an aircraft carrier ;)
You are also wisely questioning if turnover might be contributing to these outages and prolonged recovery times. Without a single doubt, again the answer is yes but it's not the root cause. Similar to how hiring changes as company grows, tactics for handling turnover has to change too. It's not like people leave the company, but within the same company they move on and work on something else. The onus is on everyone, not just managers, directors, VPs to make sure we're building things where ownership transfer us 1) possible 2) relatively easy. This in mind, veterans in these companies approach code reviews differently. If you have tooling to remove the duty of nitpicking about frigging coding style, and applying lints, then humans can indeed give actually important feedback on complexity of operations, self describing nature of code, or even committing things along with changes to operations manual living in the same repo.
I think you're spot on with your questions but what I'm trying to say with this many words and examples is, nothing alone is the sole perpetrator of outages. A lot of issues come together and brew over time. Good news, we're getting better.
Why did I move around? Change is what makes life bearable. Joining Twitter was among the best decisions in my career. Learned a lot, made lifelong friends. They started leaving because they were yearning a change Twitter couldn't offer. I wasn't any different. Facebook was a new challenge, I met people I'd love to work with and decided give it a try. I truly enjoy life there even though I'm working on higher stress stuff. Facebook is a great place to work but I'm sure I can't convince even %1 of HN user base, so please save your keyboards' remaining butterfly switch lifetime, don't reply to tell me how much my employer sucks :) I really hope you do enjoy your startup jobs (I guess?) as much as I do my big company one.