I wonder why the earth is flat.
That's strange. If the Earth were flat, then why would other people think it was round?
Everyone tells them it is round.
There was a time almost everyone thought it was flat. But new discoveries eventually proved that it was round. That's why people think it is round now.
But i've seen round things on the ground and they were flat.
I can understand why you think that makes sense. However, You should use scientific evidence to find out the truth, because not everything is how it seems at a close up look.