The metaverse is their shot at the next OS (or platform, but that's too vague). He who controls the OS controls everything (Apple for their specific desktops, Microsoft for every other desktop and laptop, Apple and Google for mobile, Google for the web - yes, the web is for all intents and purposes another platform/OS).

Facebook has their own platform but it's built on top of others so it can be choked off. Facebook wants its own OS.