Perhaps a partial answer to this question comes down to this: Private companies are treated like public entities when they reach ubiquity (i.e., omnipresence in the marketplace.) The argument is that those companies essentially become the equivalent of a public utility.
It’s easy to see a company like Google as an example of a company that has reached ubiquity. But it gets more interesting when we look at a sector—working together—that reaches ubiquity. An example is advertising, more precisely, digital advertising. Digital advertising fuels the free internet we all use. But while few companies within the advertising ecosystem have reached ubiquity, the sector overall has.