Sure, you can buy Kubota or Massey Ferguson but in many areas of the US with active farmland, you'll have to drive many hundreds of miles to even find a dealer to support these brands. John Deere is ubiquitous.
No one said farmers are not able to repair other brands.
The point is not all dealers/part suppliers are within a geographically reasonable distance. Think in hundreds and hundreds of miles or states away. Not just the other side of town.
John Deere on the other hand has rather dense distribution by comparison.
Just look at the huge list of implements here: https://www.kubotausa.com/products/tractors/utility/m4-serie...
So great, you bought a Kubota. Your nearest Kubota dealer can end up being 400 miles from you. They don't ship these things, you come buy and pick them up.