At the time that FB started writing Buck, Bazel was not open source. I believe it did exist as Blaze internally at Google before FB started writing Buck. Facebook open sourced Buck before Google open sourced Blaze as Bazel.

Over time Facebook has been working to align Buck with Bazel, e.g. the conversion to Starlark syntax so tools such as Buildozer work on both systems. I believe Buck2 also now uses the same remote execution APIs as Bazel, but don't quote me on that.