Maybe it's because people haven't heard the legends or something. When Ash saw the ho-oh in the beginning, he didn't leap in amazement at being blessed with this golden opportunity at seeing such a rare pokemon. He just looked, saw a pokemon, and questioned his dex as to what the pokemon could be just like he would any pokemon. So, if they didn't hear the legends, they won't go crazy over them.
But what disturbs me is the fact that now, instead of being incredibly hard to find/catch, you're now GUARANTEED to see these supposedly insanely rare pokemon. In the early games, you'd have to work real hard to find, hunt down and catch pokemon like articuno, raikou and the like. In the later ones, you're much more likely to find the mighty and possibly nonexistant kyogre than a dinky little chimecho or nosepass, because it's your "fate and destiny to discover this amazing pokemon" or something. Oh, sure, I feel special, because I caught palkia, just like all my Pearl owning friends have. It really makes them less exciting.