Why do some fundies mistakenly believe that Judaism is the oldest religion?

Because they are making the umption that Adam and Eve literally existed 6000 years ago and that their beliefs were consistent with Judeo-Christian beliefs, and that no humans existed before them. Archeological evidence, on the other hand, shows cavemen and even Neanderthals had religious beliefs possibly 300,000+ years ago. These were, of course, Pagan beliefs.