When you DIY that trailer hitch and something goes wrong due to the installation causing injury to others, can you held liable to a further degree because of the unauthorized/non-certified installation?
If you cook a plate of food and someone spills it on themselves and gets burned, can you be held liable because it wasn't made by a certified chef?
John Deere isn't stopping you from fixing your own tractor because they're worried about your well being in case you might be held liable for some accident. They're doing it so that they can rip you off. Any manufacturer of machinery, of any kind, would drool over this kind of captive audience. Dishwasher breaks down? It's not really yours, so you must pay the Maytag guy whatever he asks to fix it. Upgrade that video card? Dell sues you for not paying them to do it. After all, you could electrocute yourself. And now it's time to upgrade that PC to Windows 10, it's not like you have a choice here...
If this you-don't-really-own-it mentality spreads it would lead to some kind of hideous distopia. Luckily most manufacturers can't get away with it without losing their customers. Yet.