How does Samsung make money from Android, exactly?
Maybe more like they save money from using Android because they don't have to spend money developing it (aside from their own skins and customisations), and get to tap on google's cloud services for free. In short, they just focus on the hardware, while google takes care of the software (and the consumers indirectly subsidise it via advertising). That's how they can afford to give away their stuff for free.
Contrast with Apple, which spends millions every year improving on IOS, while paying for their own itunes services, plus mail, siri, imessage and icloud servers (which are key selling points, but earn no revenue on their own).
If Samsung wants to replace Android with Tizen (or whatever homebrew OS they have in mind), they would essentially need to rebuild everything from scratch, plus come up with their own cloud infrastructure and support it, in addition to working with app developers to code for their new platform. This isn't something you can roll out overnight and expect it to just work (It took Apple close to a decade to seed the ground for their IOS platform; Windows and Blackberry are now discovering that it is not that easy).
In addition, they risk alienating hardcore Android users and will definitely incur higher costs, all of which will have to be passed on to the consumer in the form of higher prices.
Android is like a drug. The costs of breaking away from it is simply too great.