They have the choice to build their own OS. The reason they don’t is clear, it’s too expensive. Google instead of charging money requires certain licensing conditions. If they truly want full control they could build it themselves but obviously they’re prefer google’s licensing model.
That's the current local optimum for phone manufacturers. The anti trust regulations exist, in part, to help prevent companies from forcing a local optimum that ensures no one ever tried something different, and thus forces the industries to keep improving.
If the OS was so good on it's own then google would not need these licensing terms to maintain market share.
