(If the earth actually is flat, what reason would the government have to hide it?)

It's because the people in the highest power are motivated by evil. The whole point is to eliminate the belief that this world was a creation by a being/God and you indeed are special and created with a purpose. If the earth is round, than you're just a creation of a universal accident and we're just flying through infinite space and you are nothing but a small nothing in the vastness of space.