Because if they don't add whatever garbage Microsoft orders them to include in their chips then Microsoft can simply require that shit for the next version of their OS to boot. They could even force an update on existing PCs to check for it. Nobody is going to buy a chip if having it means they can't run the OS that 99% of computers on the plant are using. If Intel dared to say no, MS could pretty much run them out of business.
Unless that latest chip is vastly superior to what we have today, almost nobody is going to care. Most people couldn't tell you which chip is in their computer right now. They don't even care what a processor is. They just want to be able to click on the little picture that makes facebook happen and they don't want to have to learn anything new to make that happen.
If every chip manufacturer refused, you're right that we'd be pretty safe, but the moment they can get just one chip manufacturer on board every OEM will buy those chips or go out of business. Intel was "evil inside" decades ago for a reason, so we knew how this was going to play out.