>> don't have to depend on any other software company if they don't want to.

I am rather sure that Google is heavily reliant on Microsoft and Apple to produce the operating systems used on devices that are in turn used to view Google's ads. And Microsoft is heavily reliant on hardware manufacturers (firmware is also software) to make the machines that Windows lives inside. And Apple is reliant upon a thousand and one companies that enable the innumerable mobile networks, which include many layers of non-apple software.