Why do so many people think Catholics are not Christians?

Because the Protestants are still mindset from the Reformation, 500 years ago. And fundamentalists adhere to an interpretation of the Bible that is even more incorrect than the one Catholics use, and thereby believe that Catholics don't preach the Bible. Also, because fundies don't bother to learn about the idiosyncrasies of other denominations, because otherwise, they'd realize Catholics don't worship statues, saints, Mary, the pope, or priests.