It's designed that way in the same way as a hole was designed for a puddle*. The caterpillars are evolutionarily successful because of a "spray and pray" strategy, and other species are successful because of the easy food.

Biology is an arms race, in a sense: so everything is interlinked, and affected by everything else, even if only by distant, myriad links in an unbroken web of chains. It's the reason a lot of biologists like myself are anxious about the ecological destruction that's been unfolding for so long. Life finds a way in the long term, but short term...it sucks to be alive when many of the things you depend on aren't.

*This metaphor thanks to Douglas Adams