The real question is: why aren't they? They had the infrastructure needed to seed a cloud offering 10 years ago. Heck, if Oracle managed to be in 5th (6th? 7th?) place, Facebook for sure could have been a top 5 contender, at least.
Because they make more money using their servers for their own products than they would renting them to other people. Meta has an operating margin of 41% AFTER they burn a ton on Reality Labs, while AWS has a 21% margin with more disciplined spending. Social media is a more profitable business than infrastructure.
> Advertising (over 97.8% of revenues): the company generated over $131 billion in advertising, primarily consisting of displaying ad products on Facebook, Instagram, Messenger, and third-party.