Because wherever you are, the Earth in your locality appears to be flat, and as you travel over it, it still appears to be flat. It's because, wherever you are, gravity is to all intents and purposes, acting vertically.
In history, the idea that the Earth was round was laughed at - it wasn't what people knew from personal observation. It took a very long time before the fact became universally accepted - well, by the majority of the world's population, that is.