Why did sailors think the world was once flat?

Sailors along time ago thought that the earth was flat because in their day a world being round wouldn't make sense, because they didn't know bout gravity. Their reasons were that if a world was round everything would fall off and the water wouldn't be able to stay were it was because there was nothing to hold it down. It was also very logical because when you look at the ground its flat. We now know about gravity and this answers why the world doesnt fall off into space. We have also seen the world from space and have physical proof it is round, but back then all they had as proof was that the ground is flat on your feet and that being round didnt make sense. I hope this answers your question, ali