Insects can recycle plant _waste_ into protein. I don't know if that is more effective than using it as compost/fertilizer for soy beans.

Most food is wasted directly at the producer when it does not meet some standard (size, form, flaws, color etc). Also there are many parts of plants that are not used at all (stems, leaves etc). I guess it is also much easier to grow just any plant that the insects eat, compared to something that humans like, especially in areas where industrial farming is not effective.