For one, no one has been able to unite the entire industry of installers, and never will be able to. You'll always have guys out there that will undercut others to get the work.
There are some acronyms (ABIA, RKC, etc.) working on uniting mechanics and encouraging dealers and distributors to only use qualified mechanics who are a part of some type of organization yet to be established. There's a TON of work involved in making that happen, and then getting everyone on board. And you'll STILL have guy not bound to the guidlines of the organization who will price a job at whatever they want.
You also have to figure in the level of expertise and ability (I charge more to install Simonis cloth than the guy down the street who can't put it on without wrinkles and shadows), regional cost of living differences, quantity of business. You expressed yourself that if you had more quantity, you could afford to charge less. Others take the stance that if they are busier, they can get away with charging more, or have to charge less to get the business in slow times, etc.
You also have to figure in the amount of competition in the area. If there's no one else around for a hundred miles, you can pretty much charge whatever you want. If there's 20 other guys in the book, you might have to compete for price.
There are a lot of factors involved, and we're all in a unique situation, even though we're in the same business. Besides, that's just the way things are. It's kind of like asking why don't Best Buy, Circuit City, HH Gregg, Sears and Wal-Mart all unite to sell the same model TV for the same price. Things would be a whole lot different if price wasn't a factor (and Wal-mart wouldn't exist!) LOL.
I myself am comfortable with other guys in town undercutting. It's a way of communicating my confidence in my ability to my potential customers. When a guy calls me and asks my price, then tells me that Mr. X across town will do it for this much cheaper, will I match that price, I can confidently tell him the reasons why I am more expensive than that guy. There's a reason he's had to lower his prices to get the work. You can get it done cheaper, but you can't get it done any better in this area. You'd be surprised how many people are willing to pay a little extra to be confident that the job will be done RIGHT.