If I were to "explain away" your question, I would say that it's namely because they are a species with exceptional technological capacity-- but they are too benevolent and innocent for their own good and are incapable of comprehending deception/evil. I would imagine, at the very least, they knew that the "show" had been off for a number of years and could tell through technology alone that the ship was no longer on Earth.
Ah ha, but if they were incapable of deception, why did they hide their true alien forms from the Galaxy Quest crew?
Oh god, I can't help myself, I gots ta nitpick...
SOMEBODY STOP ME !!