Well, who knows what kind of society the Power Rangers universe is in - the populace as a whole may be very closed-minded and not at all global-thinking. After all, this is the same USA that allows two teen bullies to become junior law enforcement officers, massive alien complexes to sit undiscovered in plain sight on cliffs just outside the city, and for jungle-raised wild children to be placed into society without any governmental intervention. To say nothing of all the massive urban destruction and hidden giant robots all over the landscape. Furthermore, while the scope of the danger is usually global, for whatever reason the evil guys of a given year tend to keep their assaults on mankind to one small city or town; presumably to move on from there, if multicolored kung-fu heroes weren't conveniently there to stop them cold. Sounds like the stuff of legend to me.
So, it wouldn't surprise me if as a whole, human society in PR is full of people who by nature keep their heads down and see nothing, while remaining happily simplistic and optimistic about dance competitions, science fairs and recycling campaigns run entirely by young people - the kind of Pleasantville that in the 1950s would have had lots of perfect caucasian families and young boy rascals always getting into trouble. Only our (mostly) teenaged heroes are able to see the bigger picture and, by extension, be the kind of kids that get chosen for this sort of after-school work.