Buck2: Our open source build system

I'm missing some historical context here. This article goes out of its way to compare and contrast with Bazel. Even the usage conventions, build syntax (Starlark), and RBE API are the same as in Bazel.

Did FB fork Bazel in the early days but retain basically everything about it except the name? Why didn't they just...adopt Bazel, and contribute to it like any other open source project?

One thing you might be missing is that this is Buck2.

Buck (https://github.com/facebook/buck) has been open sourced for nearly 10 years now.

The lore I've heard is that former Googlers went to Facebook, built Buck based on Blaze, and Facebook open sourced that before Google open sourced Blaze (as Bazel).

The first pull to the Buck github repo was on May 8, 2013 (https://github.com/facebook/buck/pulls?q=is%3Apr+sort%3Acrea...). The first to Bazel was Sep 30, 2014 (https://github.com/bazelbuild/bazel/pulls?q=is%3Apr+sort%3Ac...).