Traditional dealerships make their living on repairs/maintenance and electric cars just don't require that much of either
While the internal combustion engine, gearbox, and fuel tank may not exist on an electric car, they do still have mechanical components that will still need replacement... shocks, springs, anti-roll bars / droplinks, track rod ends, bushes, brake discs / pads / calipers, etc.
Electrical/electronic components wear and need replacing - fuses and wiring can become damaged or faulty through issues like water ingress. Which also means things like your water drainage needs to work, and your door / window / sunroof seals may need to be replaced. Your key (or keyless ignition system) may fail, with the key needing to be reprogrammed for your Tesla. You might lose your key and need a new one.
Teslas still have tyres; some people go to their dealer for tyres, rather than going to tyre places. Hell, even alloy wheels can get buckled if you hit a decent-sized pothole.
That's all stuff that's common between electric cars and combustion engine cars. The electric motors, the charging circuit, etc, may need to be replaced or repaired. Something as simple as a damaged charging socket would ruin your day and need fixing too.
If a chain of dealerships started carrying Teslas or other electric cars, they'd still manage to make as much money (if not more!) from repairs, servicing, and maintenance as they would from petrol/diesel/LPG models.