When Buck was created (my team created it!) there wasn't anything that supported what Facebook needed in a build tool. There still really isn't as all the Blaze work-a-likes are focusing on various different needs.

I would argue Buck actually helps to solve what you are concerned about. At Facebook, everything builds with Buck (and Google with Bazel I hear)...which means you learn it once and you know it for your Objective-C library, your Android app, your Cxx service, your python scripts, etc. It really helps to standardize on a build system, and at many companies you can only do that if it supports windows/mac/linux, it supports many languages and platforms, is fast, and is easy to pick up. Buck is all of those things.