We have a dense atmosphere.
Flat earthers have done plenty of research to prove the earth is flat, however, since we can't make this our career, resources are tight. Round earthers, however, have fewer proofs and therefore they ought try to prove their own theory. The general statement of the Flat Earth Society is "To carefully observe, think freely, rediscover forgotten fact, and oppose theoretical dogmatic assumptions." We believe everyone should follow these guidlines. Believe half of what you see and nothing of what you hear, make your own observations.