Imagine a public town square. Would you want any company acting as the gatekeeper, deciding what speech is allowed or not in that setting?
YouTube, Twitter and a few others are effectively the new town square. I don't think we want to end up in a position where a few corporations are dictating acceptable speech and open debate.
> what for many are the principal sources for knowing current events, checking ads for employment, speaking and listening in the modern public square.
The power to ban someone from social media is the power to cut them off from society. That's too much power for any private corporation to hold.