Youtube and Facebook get away with it because they're not primarily porn sites; they're social networking and the vast majority of users use it for normal non-porn purposes. Moreover, while they had vast quantities of porn, they're also huge sites and most of their users don't use the site for porn.
PornHub is just porn. And they've been on notice for years that CP and non-consensual videos are being uploaded to their system yet don't very little to stop it. They claim that they have filters and other systems in place to stop illegal uploads, but those systems are so useless that PH claimed they had 0 illegal porn on their website, when that was demonstrably false prior to them removing all unverified user-uploaded content.