I don't know what world you live in, but in my world developers don't have to look for excuses to do work. Perhaps some companies running on endless VC overhire, but the real working world has fewer employees for most positions than it needs.

And in the case of Messenger, I highly doubt it was the developers who said, "Hey, let's add this completely unrelated thing to our codebase - payments!". More likely it was marketing or "product" that told the devs what to add.