To my knowledge, Kubota doesn't produce harvesting equipment. And their largest agricultural tractor does have a lot of software (I assume it's closed source, haven't found anything to indicate otherwise).

As for running RPi or some other open-source or replacement control software, that's not a trivial thing to implement. Even if an entrepreneur releases a replacement control system, it's risky to install it (down time is expensive). Will they support the farmer? Will they be around in a year or two? Etc.