You asked for it, so here is my answer. I will try to keep it short for you.
When it comes to their cars, most people do not want to spend money. After all wouldn't you feel better coming home to a nice plasma screen TV than spending that money getting those oil leaks fixed? I mean the plasma gives you satisfaction today, right now. Fixing those oil leaks? Who cares about all the fish that die from ignoring it? You don't see them so no biggie right?
Now I just picked oil leaks out of the air and that doesn't really apply to your question so I will use a different example. Imagine this, your check engine light comes on, you can do one of two things now - 1. Take it to Auto Zone where they hook up their code reader and find code P0300, cost so far = $0. They say you need a tune up for the car. You buy new spark plugs and go home to put them in. You finish after an afternoon of work and start the car. That light is still on! What in the world? You drive back to Auto Zone and they check it again and find the same code. Hmmm, maybe you need spark plug wires huh? You buy wires and drive home. The next morning you wake up and put the wires in. Things are looking good - it only took about an hour this time! Start the car up and, whoa! That #%^@% light is still on. AAGGHHH. Back across town to Auto Zone. Guess what, same code still! They suggest that you buy this special gas treatment and run that through the vehicle. Well after driving around for a week now with that light on the gas treatment seems to have made no help. Now you are out several hours of your time, probably somewhere around $200 in parts or so. You have cuts and scrapes all over your hands, and the vehicle is no better now than when you started. You decide to call the shop and make an appointment.
2. Take it to your shop where they will need the car for the day, they will diagnose and find the root cause of the problem and tell you how much money you will have to spend to fix it. So you have left the car at the shop this morning and they tell you they will get the answers and call you before they do any repairs above a preset amount. You have agreed that the diagnosis will be no more than say, $150. A couple of hours later they call you up and say that they do indeed have the problem found. It turns out that there is some corrosion at a ground location for the ignition coil assembly. They were able to repair the grounds in the time allotted. They have cleared the code and test driven the vehicle. The problem is now fixed. You are complimented on your handy work with the spark plugs and wires but informed that you did not need either to start with since the code was a misfire but not one created by tune up parts. It was in fact an electrical problem. At least you now have new tune up parts huh?
Lets review -
1. Total spent at Auto Zone = Around $200 and about a full day of your time. No results.
2. Total spent at your repair facility = $150, without the car for a day but it doesn't matter because you went to the ball game with the buddies instead of working on your car more.
Moral of the story: Selling parts does not mean it will fix your car. As you can see by my example that no matter how many parts you bought at Auto Zone your car would never have gotten fixed. It took a trained professional with the proper knowledge, skills, and tools to find that your car didn't even need a part - just a little time to get to the root cause. Now you may think that this is a rare and not common issue and parts will almost always fix the car - wrong. Only some of the time will parts fix a car, and often times a failure in part "A" is caused by part "B" not working properly, something a code reader will never tell you.
Oh yeah - I forgot about the question. People go to Auto Zone because they want that plasma TV. The repair shop is always more expensive and they just rip you off. Or do they?
