Because they have been pressured by the US Government to engage in censorship which is unlawful for the government to perform. See also: Operation Choke Point and Operation Choke Point 2.0.

Even in cases where it might be lawful for the government to restrain the target's speech, they'd be entitled to due process and the state (or at least components of it) have found it unacceptably inconvenient to allow their targets access to due process. This 'issue' is resolved by censoring through proxy actors, and particularly through also restricting access to the relevant facts that the target would need to establish standing.