For these companies, using public GitHub isn't even a question. It would cost far more in terms of lawyers, managers, etc to approve use of public GitHub than to just buy an enterprise installation for a department.

Also, just because they have (and sell) a product that does this, doesn't mean that they use it, that it's appropriate, that their devs want to use it, etc. Separate departments in companies of these sizes act like individual companies, IBM even has the term Blue Dollars for money that they spend on other IBM services.