And yet we've seen hosts, domain registrars, and other service providers restrict certain ideas on their platforms.

How practical is it in reality to be your own ISP, domain registrar, certificate authority, hosting service, web dev agency, and community moderator all in one?

Not very.

But it's what the Constitution provides. It guarantees you the right to express your thoughts using your own resources. It does not force others to do it or make it easier for you.

The Constitution assumes a public space exists, otherwise if it was all private, there would be no point to any of the Bill of Rights, since private entities would/could override all of them. The internet is a land controlled entirely by private entities.