Your point is well taken, however I don't think that the majority of cruisers know or care that Celebrity is "supposed" to be more formal. Most people don't spend time researching dress codes on message boards prior to booking a cruise, rather they choose a cruiseline based on other factors such as itinerary, price and availability.
Your post highlights one of the major problems that we face as a society today -- the problem that many people do not take responsibility for their own actions. If you are buying a product, you bear the responsibility to know what you are getting and to choose a product that is suitable for your purpose. Alas, the way you and your coworkers check out cruise lines sounds like the guy who bought a small pick-up with a four-cylinder engine and then wondered why it would not pull his Airstream at highway speeds.
It is not very difficult to do a bit of research on cruise lines and ships before one books one's first cruise. You can walk into any major bookstore (Barnes & Noble, Borders, etc.) and find about half a dozen books that evaluate and rate cruise lines and cruise ships in the travel section. Most of those books say very clearly that there's a profound difference bttween lines. Further, it's awfully naive to assume that all cruise lines would be carbon copies of one another when houses hotels, restaurants, automobiles, and even supermarkets and department stores clearly are not.
If one books through a travel agent, the travel agent also has some responsibility to assist the client in choosing a suitable cruise line. If I were a travel agent booking a cruise and a client seemed unfamiliar with the industry, I wuold specifically ask questions about basic lifestyle thta would help me to match the client to a suitable line. Failure to do so would be a disservice to both the clients and the lines who would end up mismatched.
I know my cruises on Celebrity didn't seem any more formal than on Princess. Informal nights looked alot like Princess' "smart casual". I don't think many people realized there was supposed to be a difference. Formal nights on both cruiselines were pretty much the same with a wide variety of dress from ultra-formal to dressy casual.
Are you really that blind?
After thirteen with Princess six cruises with Celebrity, I see the differences pretty clearly. The experience on Celebrity may be somewhat different if you take very short cruises in the Caribbean, but Celebrity's cruises are much dressier than Princess's cruises even in Alaska.