Governments have vastly more power over their citizens than any corporation. Facebook cannot imprison or kill users that violate their rules, and being banned

Governments have vastly more power over their citizens than any corporation. Facebook cannot imprison or kill users that violate their rules, and being banned from Facebook is not all that big of a deal (I have not had a Facebook account for more than a decade, and honestly I feel no particular need to go back). Moreover, if you required private for-profit corporations to adhere to the first amendment, you would quickly run into problems. Scientific journals could not exist -- the first amendment comes with no requirement for scientific rigor. Newspapers could not exist -- the first amendment has no requirement for journalistic quality (hence the legality of fake news). The framers of the constitution could certainly have included a requirement that private institutions follows the same rules as the government. There were certainly big and powerful non-governmental organizations at the time -- the Catholic Church, for example.