John Deere sells tractors because of their oncall process

When you're in the middle of harvesting and your tractor breaks down, you want it fixed now, not at somebody else's convenience.

Mechanics and tow trucks also form an oncall for broken down cars.

There's oncalls all over the place for tech of all kinds. I think the biggest difference with software is intellectual property - we've made it so nobody else is allowed to fix whatever's broken, so of course, we need oncalls to fix the problems instead of letting customers go to their preferred mechanic