Here's why:

* Stars are spread throughout the galaxy: Our galaxy, the Milky Way, is a vast, three-dimensional structure. Stars are distributed in a spiral shape, with a central bulge and arms extending outwards.

* Perspective: From our vantage point on Earth, we see stars spread across the sky, but this is due to our perspective. The stars are actually at vastly different distances from us.

* Distance: Some stars are very close to us, while others are millions of light-years away. This vast range in distances means they are not all on the same plane.

Think of it like this: Imagine you're looking at a forest from afar. The trees appear to be on a flat plane, but in reality, they are at different heights and distances. The same concept applies to stars in the night sky.