Traditional social media is run for-profit and thus has an incentive to keep their website online as much as possible to keep their company alive by gaining users and revenue. And I would bet they do have backups. Hobby websites like fediverse projects often are can be run by any flaky nobody that can have varying motives and varying data retention practices.

They did.

There's almost hundreds of dead social media that inaccessible, some even have zero backups. https://en.wikipedia.org/wiki/List_of_defunct_social_networking_services

Usually they have VC money to burn through