If a person wants to take openness to new ideas to its logical conclusion, she will not end up being gullible or credulous. Rather, she will be a skeptic. True open-mindedness is not the same as accepting assertions without critical consideration. In fact, believing claims without being critical of them results in having a closed mind.
Why do people believe in ghosts? You'd think that if they'd existed all these thousands of years, someone somewhere would've been able to get positive proof. Yet all we've come up with so far are a million anecdotes and those "reality" ghost shows, which are basically just footage of a bunch of idiots running around in a dark house and asking one another, "Did you hear that? Did you see that?" Yet a lot of people still believe they exist. And not just ghosts, but manananggal, tiyanak, mangkukulam, and a host of other supernatural beings.