Worms *do* eat the OM, at least the ones in my bin appear to ;-) They simply don't eat it for the OM itself, but for the bacteria coating it. Kind of like me when I eat hamburgers. All I really want is the slab of meat and tomato slice inside the bun, but I still eat the bun because the meat and tomato are inside it ;-) This is why fresh food is ignored in a bin, but as soon as it starts to rot at all, it gets gobbled.

Beyond that I don't have any ideas why a worm's digesive system is why it is ;-)

worms are omnivores and do have crops and do need grit [very fine] because no matter how small the particles of food they ingest [microbe or veg], it still needs to be ground up, and they're about 80% water, 0% teeth - the organic material they take in is actually nearly liquid, being decomposed by the microbes - so they don't munch up the food scraps you give them, they "slurp" the surface off repeatedly as it rots, until it's gone

Bill