Dagobah is flat, you silly!
And, duh, clearly the earth is flat, since I can see my truck on the other side of the parking lot. If the world was round, my truck would disappear behind the pavement stripes!
—
Actually, it makes sense that this is how it all got started. Someone made a flippant joke, someone else spread it along until it landed in the lap of a Hobbit hiding in its hole, and then the Hobbit got all hot-n-bothered by the "conspiracy" and ran off to Mordor to destroy the big, bad, Mason-Klingon, whilst dragging along a bunch of innocent—but gullible—buddies and putting their lives in peril for save Padmé!
And Star Wars is
real, dammit!