The issue isn't expanding data centers -- that's "easy". The issue is all of the software tooling, all of the gigantically massive platforms and services they have to not only build, but catch up to AWS/Google/Azure, all of which are moving targets.
AWS launched in 2002. Google Cloud (GCE) basically started in 2012 and only recently has finally mostly caught up to become a fully-fledged viable competitor, which took an entire decade. Meta would be too late to the party.
Sure they could launch something much smaller like a Digital Ocean competitor -- not a fully-featured cloud, mostly just servers and storage. But why would they? The margins are low and there's no strategic reason to do so.