It's because we take a zetetic, not a theoretical approach.
Contemporary scientific method (which has arrived at Round Earth conclusions) tends to begin with a hypothesis which is later tested in attempt at validation. If a scientist has a conclusion in mind before he even makes observations, how can he hope to avoid subconciously formulating evidence and theory which supports it?
Zetetic science puts the observation and testing before the hypothesis - any Flat Earther worth his salt will have addressed the issue of the Earth's shape without any preconceptions, and concluded by observation that the Earth is flat.
Most Round Earthers will simply have learnt that the Earth is round from higher authority (the scientific establishment, the media) and will not have sufficiently verified the "facts" for themselves.
The Flat Earth Society does not encourage people to believe that the world is flat, it encourages people to find out the shape of the Earth for themselves. Why do I believe? Because I did just that.