However, this is an orthogonal issue to not reviewing code. To contrast, if we push code to production _before_ review, then production is much more likely to be full of bugs, and we force people to go looking for them afterwards. I've worked in this kind of environment, and the amount of wasted resources (both compute and engineering time) is staggering. Somebody's pet project ends up taking way more compute than anybody would have deemed acceptable and we have exactly the overhead and waste you are referring to.
Better, lower overhead abstractions require buy-in, and this means it needs to work for other people than the person who wrote them. At some point it needs review and discussion, otherwise it will just be a continual edit war between competing parties.
As others have pointed out: code reviews exist for many complex projects even outside of these big companies. Code reviews existed before these companies.
I mean, how have we ended up in such a situation as today, where were have--in our pockets--a computer that would have been considered a supercomputer when I was born, and yet all it does well is make sure advertisers can reach me?
You want to defend the practices of major engineering organizations? Get ready to defend their business practices, too.
Yes, I know I'm coming off as a blowhard. But I think if you really start to look at the technology industry, try to figure out its ills, you start to realize that you can't unpack the good from the bad. That ethics are way out the window, because engineering erroneously sees itself as separate from business. "It doesn't matter that Meta makes it's money off of manipulating users into purchasing decisions, I'm just working on a web app framework". Our engineering jobs would exist without the business unit. Whatever we do, no matter how far removed from the business it is, our jobs wouldn't exist if they didn't count to the bottom line.