"And yet phone companies and postal services aren't liable for what people send using their services. "

Postal service does not read your mail, phone companies (are not supposed to) listen in on your calls.

'Internet companies' deal directly with the content of the material.

Notice that 'email hosts' are not the issue here.

It's Google/FB/Twitter etc..

Not taking any position on the issue, but this is a rathe salient difference.