Thomas Shey
Legend
Not generic in one sense, true. They are very recognizable. They are also not bound to a specific world - the classic D&D dragons for instance have appeared in the Faerun, Greyhawk, Mystara, Dragonlance, Dark Sun, and even Spelljammer settings. And while there are differences (especially with Dark Sun and Dragonlance!) a Green Dragon is still a Green Dragon in each of them. Moreover, the differences the settings may apply are not detailed in the main, non-setting specific, rules.
Likewise for Mind Flayers and Beholders. And Displacer Beasts and Owlbears.
Basically, it's a case of keeping the crunch away from the fluff, and Paizo does a poor job of it. The counterpoint, of course, is that Paizo also doesn't contend with multiple settings, and doesn't intend to. In fact, with Organized Play such an important part of their corporate strategy, they're actively resistant to supporting multiple settings. So it's a much less important issue to them.
Well, you feel how you feel, but frankly, fitting beholders into multiple settings doesn't seem any more or less an impediment to homebrewing than the fact the Paizo monsters have some attached setting cruft. If I can get around one I can get around the other.
For the record, you are aware that the "classic" D&D dragons are designed around the 5 Europeean Renaissance/Alchemical Elements, yes? Air, Earth, Fire, Water, and Spirit (or Aether/Ether), right? So not really all that distinctive a concept.
I'm not sold they really are. It strikes me as much more likely it was a case of making visual differences and then doing the associations with those colors to come up with breaths.