As pointed out this isn't what happens, but even if vendors did ship Coreboot it'd have the same behaviour (because that's what the vendors want) and you still wouldn't be able to reflash it because they'd still be using Boot Guard. Coreboot isn't some magical freedom enhancing technology that can be sprinkled on a platform to make it respect its users, in the same way that the use of Linux in Android doesn't guarantee any strong user benefits over the iOS kernel.