Because the more general the rule is the easier it will be to enforce it. To stay with the example if there is a rule against auto shops, and you run what in effect is an auto shop but you say you are not taking money for it can we shut you down? Will we need to go to court where expensive lawyers battle it out what it means to be running an autoshop? That is a lot of cost and even more uncertainty.
If the rule is about not being allowed to work on cars at all, then all they need to show is that you worked on a car. They don’t have to care who owns the car, they don’t have to show if it was commercial work, they don’t have to show if it was a regular activity or not.
(Plus, just like you usually don’t get pulled over for 75 on the highway, most everyone will be able to do occasional work on their adult kid’s car anyway.)
But neighbors can be both difficult, and ever-present.
I had a busy-body neighbor once that would go out of his way to make sure that everyone was following the rules.
For instance: It'd been snowing, quite a lot, and the roads were officially closed. But I had to get my wife to work at 5:30AM anyway: Those road closures don't apply to people in her line of work in the medical field.
So we went out early and started digging. We thought we had made a good path but got the car stuck in the middle of our boring little side street anyway. Fun times.
And the nosy neighbor dude waddles up from his house. He doesn't offer to help, like many normal neighbors might have done, or a great neighbor would feel compelled to do.
His compulsion was different.
He just takes a series of photos our clearly-evident (to him) lawbreaking activities and smugly waddles back inside.
This is the kind of neighbor who absolutely would make sure I would be reported for working on a non-resident's vehicle in my own garage.
I've only had one such neighbor in my decades of adulthood, but he cannot be the only one who is this way.