They may seem like they have the whole world, but they really don’t. They’re constantly under attack from just about everything, and they’re extremely specialized to certain food sources and reproduction methods (only able to lay eggs on one specific plant or whatever), so in reality they’re often confined to fairly small areas based on climate conditions, food availability, and predators.

One of the toughest things for insects to deal with is a closely related species that is simply better adapted to an area. There’s no way for the less adapted species to move in because it’s getting outcompeted in its own niche.