This is an interesting question.
Asking why is earth the only flat planet betrays a logical fallacy. Karl Popper talks about this in Logic of Scientific Discovery, and its a well covered topic in philosophy of science, presenting one of the largest issues with empiricism.
As it turns out, each planet we view that is round actually makes it more probable that the earth is flat - this is one of the issues behind the problem of induction. Since the earth being flat has a non-zero probability, each time we observe a round object it reduces the solution space, thus making the non-zero probability have a higher likelihood.
More than this, the idea is logically faulty. I may see a grey goose out my window each day. Does this mean all geese are grey? In other words, since I can say Object A and Object B are both blue, does that make Object C blue? or Object D? Or all other objects?!
Have you even thought about this at all? Even at a passing?