Easy. Constitutional rights do not apply to private corporations. State action doctrine.
I wrote a law review article on this topic. SCOTUS has rejected that analogy in multiple modern attempts to apply that exception.
Here's an in depth analysis of the topic.
Actually in this case, the current Court would be far less likely to rule social media companies as a state actor. Originalists would not flexibly apply the doctrine to apply to modern facts.
lol. i'm a lawyer. you're referring to legal philosophy, not the law in practice.