Also keep in mind they've been in hypergrowth mode since 2010 or earlier. Regardless of whether Git is the clear solution today, it definitely wasn't at that time. So it makes sense they invested in building their own tooling.

And now it's 2023, and they're inventing the wheel again.

Not a very robust analogy. There are tons of different wheel designs out there. Some are good for racing and suck for rain. Some are great in the snow but are noisy and inefficient on the highway. Etc... Maybe their needs are so special they need a wheel design that doesn't commercially exist?

Someone really needs to reinvent that analogy.

You might be surprised to learn that they're solving the wrong problem.

I don't think you fully understand how big their codebase is and how many different teams are working on it at any given time.

There are very fundamental differences between a mono-repo and a bunch of repos for each "service" or whatever. Lots of tradeoffs. I've worked both and I can see the reasons for huge monorepos. They make a lot of things that were previously hard much simpler... The tradeoff is your tooling needs to be able to scale with growth of the company. And for a company the size of FB, dedicating an entire team to improving the tooling for their monorepo is well worth it.

Sit down. I worked at FB.

One year in 2014/2015?

Saying that Meta "reinvented CVS" means you either know very little about source control or you are just being purposely misleading. Neither is a very good look.