Here's why:
* Large mass: The immense mass of these planets exerts a strong gravitational pull, holding onto their atmospheres.
* Strong gravity: This gravity prevents gases from escaping into space, even at the relatively high temperatures found in their upper atmospheres.
In contrast, the terrestrial planets (Mercury, Venus, Earth, and Mars) are much smaller and have weaker gravity. They lost significant portions of their original atmospheres due to the escape of lighter gases into space.
