While it's true that rotation can cause a planet to bulge at the equator and flatten at the poles, this effect is not strong enough to make a planet truly flat.
The Earth, which rotates relatively fast, has a slightly oblate shape, meaning it's wider at the equator than it is from pole to pole. However, the difference is only about 0.3% – not enough to be considered flat.
The reason why planets don't become flat is due to their gravity, which pulls everything towards the center. Gravity is a much stronger force than the centrifugal force created by rotation, and it keeps planets from being significantly deformed.
It's important to note that there are no known celestial bodies that are perfectly flat. While some objects, like some asteroids, may have irregular shapes, they are not considered "flat" in the way you might be thinking.
