I used to go to a mechanic that my family knew for decades. He's still around and is still one of the most honest businessmen I know. The problem was his shop wasn't advancing at the same rate cars were. For example, he couldn't mount tires on wheels larger than 15"; he had to send them out. When I got an imported luxury car, he often had trouble getting the right parts. It just started to make more sense to me to take it to the dealer for service, especially for the high line brands.
The other thing is, dealers don't often try to pull the wool over the customers' eyes anymore. With the rise of the internet, it is so easy for consumers to find out what things really do cost and how things really do work, it doesn't benefit dealerships to be ripoffs. Having worked at an auto firm, I can tell you the dealers WANT to satisfy customers and have them coming back and give glowing reviews. Partially because they really do want to help, but also because the better they treat a customer, the more likely they are to come back. Also, the more positive reviews they get, the more co-op dollars they get from the manufacturer.
Service and parts are where car dealers make a majority of their profit. The other majority is F&I (the office where you sign the final paperwork, where they try to sell you LoJack, paint protectant, extended maintenance plans, etc). When it comes to service departments they really do want to help you because it's also in their best interest to do so.
Private mechanics are not governed by the same principles, so there can be honest good guys and lying bad guys.