Why is it flat? That I cannot answer. I would say it formed that way, but then I could also say God created it that way, though I'm not a creationist.
The government's only discovered the true shape of the world in the 20th century, and with all the science and history stating the earth was round, they knew that to reveal it as flat would rewrite the science books. NASA cover it up now, but low level scientists aren't aware of the deception. Even Google are in on it, with their Google earth. In fact, a high level employee recently got suspended for coming out and saying earth is flat. It's not just about science though, its about control. It's great to make people think elon musk launched a tesla into space. It's great to make people believe the curiosity rover is roaming mars. It's great to make people believe we are looking at space tourism with companies like virgin galactic. It's great for them to make people think that a base on the moon is something being looked at. It's great for them to talk about asteroid mining.
The truth is, earth is flat. Dr Samuel Rowbotham proved it in the 19th century, and despite desperate attempts to airbrush him from history, they failed. His work lives on, and we are his 21st century followers. If you read earth not a globe you'll find a wealth of knowledge and science that cannot be disputed.