The difference is that the tyre manufacturers don't pay Ford to stick their tyres on Ford's vehicles.
They're not paying Ford for putting a Michelin tire on the vehicle, but they are likely getting an above "volume order" discount.
Similar to Search engines, how often do you hear about people changing the brand of tire on their vehicle? How often is it just "I bought another of what I already had"?
Disclaimer: Former Googler, though in Cloud.