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.
Also how exactly they would do it, they don't have enough infra for renting, they would need to x10 what they have now.
Offering reliable IaaS is super hard and capital intensive. Its also not profitable if you are perceived as shit.
Google started a cloud and their user-facing software is atrocious. Compared e.g. Angular to React, Tensorflow to Pytorch.