I think we know why. I've seen spiders carrying off caterpillars when I am out late at night with a flashlight gathering leaves, wasps chopping them up, assassin bugs sucking the life out of them, lace wing larvae, too. Japanese ladybugs will eat them both eggs and larvae. Mice, snakes, birds,... Then, throw in the hot or stormy weather. The birth of a butterfly is truly a miracle.