Because they are not produced for human consumption. They are given growth hormones to grow faster and given the food scraps of bakery's to eat (Moldy bread, bad bread, ingridents, ect, ect. also poop and just st right dirt.)
I never liked the idea of using earthworms as feeders as they are "sticky." I was asked by a local worm farm if I would like to use these and checked with UC Davis Vet School. They said not to use these as they have a lot of soil-based contaminants like selenium (have locally). Fine with me!