Here's why:

* Planets move: Planets like Mars orbit the Sun, and they constantly change their position in the sky relative to the constellations.

* Constellations are star patterns: Constellations are simply groupings of stars that appear close together from Earth. They are not physical groupings, and planets are not part of them.

So, while Mars might appear to be in a particular constellation at a given time, it will move on to another constellation as it continues its orbit.