Because they are very expensive.
Usually, only people with new or govt./company maintained cars go to these dealership. The average user prefer the road-side ustaad and before these electronic engines, the ustaads were pretty reliable as well. Even today, unless it involves fiddling with the engine, an experienced (and not adventurous) ustaad can probably do a better job at half or one-third the price.
I took my company maintained family car (Cultus) to 3S 3 times to get locks fixed and A/C service. It will develop the same problem every 3-6 months. Took it to I-9 ustaad, got it done at a cheap price, and no worries since.
Some 3S are a true rip-offs.