Here's why:

* Composition: Gas giants are primarily made up of hydrogen and helium, which are light and gaseous. Terrestrial planets, on the other hand, are composed of heavier elements like rock and metal.

* Gravity: Gas giants have enormous gravity due to their immense size. This gravity holds onto their vast atmospheres, preventing them from escaping into space.

* Size: Gas giants are much larger than terrestrial planets, meaning they have a larger surface area for their atmosphere to spread out over.

In summary: Gas giants have thick, extensive atmospheres due to their composition, powerful gravity, and large size. Terrestrial planets, in contrast, have relatively thin atmospheres.