They charge for the diagnostic systems. Bigly. For example, Mercedes-Benz's Star Diagnostic System (SDS) is necessary for a variety of repairs and diagnostic procedures. There are varying degrees of workarounds and alternatives but none of them work quite right, or for every model/year/variant. It's not just the embedded system, it's also the interface to it. That's where the really ugly rent seeking crops up. And that's precisely why a tractor with no computers is attractive--not because the embedded software might try to ransom itself (although that's a reasonable fear) but because some horrible rent seeking corporate functionary will do their utmost to cheat you (or your mechanic) out of as much money as possible when it comes time to do any maintenance or diagnostic testing. No computers means that little bastard can fuck right off.