What's the short version of why Meta did this, wrapping so much up in this internal nonstandard format?

I'm guessing same to why everyone else does that: they want to abstract away the platform. To some people platform intricacies are uninteresting comparing to the problem they are solving, so their abstractions aim to express their problem domain, and hide everything else.

For example for apps you may like to deal with abstractions that can express navigation and pages. They are the same across web, iOS, Android. Unlike Activity, Fragment, View (is that still a thing, I stopped doing android 10 years ago lol) and some slightly different set of abstrations over iOS.

Often, performance.

When I was at Meta, if you could measure it, you could ship it and mark that as a win in your performance review. So a lot of projects got shipped off of A/B tested metric wins.