As an industry, it’s important to teach new generations.
A senior engineer with good self discipline can work from home.
A great engineer will elevate their team and thus be more valuable than working alone. And this process helps juniors grow.
You could learn to be a mechanic from books and then have a service that matches people with car problems with mechanics at home. Why do you need a work site?
More often than not, these things have to take place at the shop because there are specialized diagnostic and other tools (like a mill or crane like OP says). These are not portable pieces of equipment.
So your analogy sort of falls flat there. It would be like the server master (made that up just now and I like it). That person may not be able to work from home, because the physical machinery is important.
Otherwise, yes, your point of teaching youth is important.
I can think of 3 good reasons:
1. Businesses don't want to remember a bunch of addresses and who sent what where. Even if it's dropped off at a central lot, you'd still need to arrange further transportation from the lot since a lot of the equipment is broken, doesn't even have tires, or is otherwise not street-legal.
2. Diesel mechanics don't usually work on consumer diesel engines - this is heavy equipment. These shops tend to be located along highways for a reason - they need the wide berths highways provide. You'll have trouble fitting a lot of this equipment in your driveway, much less your garage.
3. The shop insurance situation would be a nightmare.