I share your intuition, perhaps unfairly, that it's indeed not as hard in absolute terms. However, it certainly requires a different set of skills and organisational practices.

Just because an organisation is extremely good at one thing doesn't mean it can easily apply that to another field. I would guess that SpaceX probably does have the talent on hand to throw together a Facebook clone, but equally I think they would struggle to actually complete with Facebook as a business at scale.