For example, consider a small ant on the large roof of the Tokyo Dome. The roof of the Tokyo Dome is curved when seen from a distance. However, to the ants on the roof, it should look like a flat surface. This is because the dome is too big for ants.
It is the same thing that makes the earth's ground look flat to humans. It is only because the earth is so large compared to humans that it appears flat even though it is really round.
By the way, although the earth looks flat, there is a way to confirm with the eye that the earth is still round. Watch a ship approaching from far away in the ocean.
At first, you cannot see the ship at all because it is on the other side of the earth's sphere. However, as the ship gets closer, you can gradually see the top of the ship, such as the coils and masts. If the earth were flat, then the blurred images would become clearer and clearer. However, because the earth is round, the shape of the ship appears from the top of the ship. This is good evidence that the earth is round.