That's because people don't understand there's more than one model and if they do they don't understand how to tell them apart or what the benefits of a new model are.
That's just not correct. Look at the ebay listings. Every one features the model number and tech specs right up front.
Plenty of valid criticism can be leveled at Apple, but are you really suggesting that they aren't doing enough to announce their laptop hardware revisions?
They're not exactly shy when they update one of their laptop models. They tend to have a big media production where the CEO gets on a stage and explains the hardware and software changes in some detail. Convincing people to replace their 2009 macbooks with shiny 2014 models is a big part of their business. I'm not sure how much more they could do to spread the word that "this year's macbook has a better SSD/CPU/whatever."