Here's why:

* The Earth's Orbit: The ecliptic plane is defined by the Earth's orbital path around the Sun.

* Other Planets: While other planets don't orbit in exactly the same plane as Earth, their orbits are very close to it. This is because they all formed from the same protoplanetary disk, which was initially flattened.

Important Note: While the ecliptic plane is the dominant plane for planetary orbits in our solar system, it's not perfect. Planets have slight inclinations (angles) relative to the ecliptic, and there are also objects like comets that have highly inclined orbits.