The reason people don't fall off the "bottom" of the Earth is due to gravity. Gravity is the force that attracts objects towards each other. On Earth, gravity pulls everything towards its center, including people.
The Earth is spherical in shape, so regardless of where you are on its surface, you are always being pulled towards the center of the Earth. This gravitational force acts perpendicular to the surface, keeping everything on the Earth's surface, including people, firmly in place.
Imagine standing on a ball. You are pulled towards the center of the ball, and no matter where you stand, the force of gravity keeps you on its surface. Similarly, on Earth, gravity holds us to its surface and prevents us from falling off.
It's worth noting that when we look at the Earth's surface, it appears flat at small scales due to its large size. However, when considering larger scales or observing phenomena like the curvature of the horizon or the shape of Earth's shadow during a lunar eclipse, the spherical nature of the Earth becomes more evident.