If the linux community as small as it is can produce multiple varied and unique linux distributions largely on the backs of volunteers, there's no reason why these manufacturers (especially some of the bigger names) couldn't do the same with Android / Linux and their own hardware. And whatever reason is behind the failure of literally the entire cellphone industry to do what they were doing before the advent of iOS and Android, it isn't because Apple is somehow stopping them from making their own OS, and SDKs and app stores.
Can you buy the display from a supplier that supplies Apple and put together your own phone? No, they have exclusive agreement with apple.
Their anticompetitive practices Make It incredibly difficult and complex to put together a device. That's the whole point!