system
Given the following post, respond as if you're commenting on Reddit:
human
[Dungeons & Dragons] Why haven't the Inner Planes invaded and overrun the Prime Material?

All the Inner Planes have a good reason to invade, because the Prime has a huge amount of resources that are rare on each of the Inner Planes. Creatures from all of the Inner Planes are extremely powerful compared to creatures on the Prime, so it wouldn't a military problem. Travel is really easy because the Inner Planes are the third-closest planes to the Material (right behind the Shadow and Ethereal). So why are Lower Plane and Far Realm incursions more common than elemental?
gpt
Because MOST Elemental entities aren't that bright. And those that are tend to spend more time trying to maintain their possition than expanding. 

The Elemental Planes are barely contained chaos. They exist in a tenious equilibrium between each other that constantly forces the ruling powers of each to invade their neighbours and defend their holdings, lest one element rise above the others. 

Any venture into the Prime Mateiral Plane comes at great risk, because while technically infinite, the power of each Elements expression is finite. If Fire invades, then Water gains an advantage in the struggle between the inner planes, and Fire's forces may not return in time to reestablish the ballance.