May's also seen Ho-Oh or such, haven't they? I mean, flaming, dangerous things in the sky can usually be traumatic to people, and so they might just be being reassured for that, but anyway. Anyway, generally it's sort of assumed that seeing legendaries would be not that uncommon if you were around their regions, and Ash does generally travel around quite a bit. Team Rocket, for instance, who follow these people around, have probably seen a fair few legendaries as well, which might make their journeys more useful as otherwise them being sent around to look for a Pikachu constantly might seem strange. In general, though, being able to see the legendaries - recall that they didn't remember Mewtwo after M01, humorously being afflicted with amnesia - isn't treated as that different, and many trainers or non-trainers do this, but their stumbling into the same path is usually in part a result of journeying often, and then of usually straying from their overall journey a fair bit in order to engage in plot interactions elsewhere, which others such as Gary - who catches rarer Pokémon - or Tobias are generally not shown to do. There is a certain, queer case of 'ball watching' there, or they tend to focus on noting and seeing Pokémon rather than battling them - in this sense Gary Oak might as well have just tagged along instead of having to abandon training for 'research,' and then could have had a Contest sub-plot and replaced May's role in the team and everything would be better. As in Tobias' case, it might be assumed that others can run into legendaries significantly more often, of course.
As has been pointed out, it is strange that after continually seeing these legendaries, they have never tried to catch them. That said, it might be in part because they only see them as a part of detours or plot strays beyond their own journey, and hence unlike Tobias for instance couldn't really integrate them into this journey in the same way, and had to leave them there.
