Unfortunately raw grain can be surprisingly hard to make edible. For oats for example every single individual oat has a shell that's inedible. This is different from the bran that is edible. You would have to ground the oats up and sift away the bran. And then you are left with oat flour and not rolled oats. I would say rolled oats are impossible to make at home.
Most people want to buy pasta, bread, white rice, not raw grain. Unfortunately for the farmers the processing of grain is a more profitable business than actually growing the grain.