30 years is a long time. A whole generation is born and has become adults. Some of them might already have children. And Order 66 was even 20 years before that.
The memory of the Jedi has faded. I'm often astonished about how little young people know about history - in my case, about the struggles in Germany after WWII, about the division of Germany in two and about the struggles for families that were on both sides of the border. Just this year it was the 30th anniversary of the re-unification, and the memory is already fading -- in an highly interconnected world where media access is ubiquitous.
Rey is living on a third-world planet, is poor and struggles with making enough money for food. It is surprising she has heard of Luke at all. And its really not surprising that she takes the stories about this great hero who she never saw, who can allegedly do magic with a hefty grain of salt. She has never seen anyone do magic, why should she believe that it does actually exist? Because from her PoV, "The Force" is nothing more than a fancy word for "magic".
Remember what Admiral Motti said?
Admiral Motti : Don't try to frighten us with your sorcerer's ways, Lord Vader. Your sad devotion to that ancient religion has not helped you conjure up the stolen data tapes, or given you clairvoyance enough to find the Rebels' hidden fort...
[Vader makes a pinching motion and Motti starts choking]
Darth Vader : I find your lack of faith disturbing.
Vader is a prominent figure in the Empire, to put it mildly, and that is in EP IV (BBY 0), so Order 66 was executed only 19 years prior. Motti is clearly old enough to remember the Jedi and Order 66, and even he doesn't really seem to believe what Vader can do -- prompting the latter to use Motti for a little demonstration.
Or what Han said in EP IV?
Han Solo : Hokey religions and ancient weapons are no match for a good blaster at your side, kid.
He clearly doesn't believe The Force is real -- and he is well travelled and a lot better informed than Rey.
Luke also wasn't much of a public figure. He was a pilot, not a general, unlike Han, who got promoted to that rank prior to the Battle of Endor and went on to become the husband of Leia Organa, who played a rather prominent role in the Rebel Alliance and later New Republic.
@Juhasz puts it very nicely in a comment:
Yes, he fired the photon torpedo that blew up the first Death Star - but without looking, what's the name of the pilot who dropped the atom bomb on Hiroshima? He was a guerrilla fighter in the battle of the second Death Star and was present for the death of the Emperor (though there were probably no living witnesses of this). Why would anyone aside from a history buff know any of this?
Given these facts, and that its 30 years later, I don't find it surprising at all that Rey thinks Luke is a myth. If I had only her information, I'd probably think so, too.