To think the earth is flat one must disable logic and reason. This has a wider effect than just the shape of the earth. The people that suffer from flatearthism also are fooled by other things that can cause harm to themselves or others.
Why do they believe it so much and hold to it so dearly? When one accepts something absent of logic and reason, it is a religious belief. These people then feel the need to proselytize their religion. This is common throughout human history. The difference is that most religions have spiritual beliefs about spiritual things. Something that is not scientifically testable. Flatearthism is a spiritual belief about a physical thing. Something that is scientifically testable, and has been confirmed over and over for centuries.