Let me guess, you're an IT amateur...
"detach a cable, slip a sleeve on, and reattach it?"
You've missed out checking pre-requisites :
Check the car's serial number against affected models.
Check car hasn't already been fixed.
Visually inspect existing cable and decide if it needs replacing or patching.
Test entire system for faults so you can be sure that you haven't caused any new faults.
I'm sure I could think of a few more if I could be bothered.
Then, the "detach cable" part doesn't include the time where you have to extract it through a hole about 2" diameter hidden underneath some other panels that require you to remove half the car to get at.
Similarly, if you've bought a new car recently, you might have seen that headlamp bulb replacement is something that is a "main dealer" only task. They'll probably bill you an hour or so's labour just to change a bulb.
At the end of the day it's probably just that an hour is the smallest unit of time their maintenance process system can cope with. "Check your oil level sir? Takes an hour.", "Refill washer fluid sir? Takes an hour" etc.... "Charge your battery sir? Takes a week."
(I know a Tesla won't have engine oil, but presumably it still has gearbox oil.)