Everyone subjectively weighs evidence in order to come to a conclusion. For many people this is simply a matter of hearing whatever "the expert consensus". That's fine, but once you realize that knowledge is power, and people can be manipulated for selfish ends, one starts to question things a little and do their own thinking. I'm not suggesting that everyone who believe ball earth hasn't done their own thinking or research, but probably the vast majority of people just believe what they are first taught.
In the case of the shape of the earth, IMO the globe advocates have the onus of proof, as nothing about our daily experience supports the extraordinary claim we are on a large spinning ball: We can't see any curvature, we can't feel it's motion, we can only theorize the existence of gravity (We can debate the Cavendish experiment), we can't demonstrate the supposed perfect adherence of water to a spinning sphere, the existence of a pressurized system (Earth's atmosphere) next to a vacuum, etc. On the other hand, every natural phenomena that I am aware of has adequate alternative explanations, whether eclipses, the movement of the sun, the rotation of the stars, the seasons and so on.