> Why should we consider them public spaces when they are private companies?

As the other commenter is suggesting a change to the status quo, what legal status they have now isn't important, but instead what matters is role they serve in society.

Personally I think the US idea of freedom of speech is too anarchic to be sustainable, but even with that, the de-facto power that Big Tech has over communication (and commerce) means I think Big Tech should be held to a similar standard in this regard as any government.