Being able to get service in the field quickly is critical during planting or harvest. During these times downtime can be exceedingly expensive if the farmer is trying to beat some change in weather.
And sure you could try and get someone other than the dealer to work on it, if you can find them. But trying to find this person when your equipment is broken and you are burning money isn’t a great option and the dealer will probably favor servicing clients who bought equipment from them.
Boeing actually seems to have more diverse competition, including Lockheed, Northrup, etc (although they're dominant in the commercial airplanes segment, where airbus isn't mentioned, so maybe these stats are US market only): https://csimarket.com/stocks/competitionSEG2.php?code=BA
John Deere manufactures it’s own engines in many cases, but it also uses engines from other manufacturers.
> never (intentionally) leaves the ground or exceeds 30mph
Yes, aviation has a lot lower risk tolerance than agriculture, but ag equipment still kills plenty of people, and I’d probably rather be in a head-on collision with a honda civic going 50mph than an 8R going 30mph.
> Most medium sized countries have tractor factories, as they're essential for food production for the population.
Are you just reiterating that John Deere has more competitors? If so, I think this is misleading because most of these factories are just the same few competitors with factories in different companies (rather than distinct companies per country).