Because ivermectin's an animal wormer, not really used on humans and it can get into the eggs. If you're sensitive, it could be a problem. (Personally, I'd probably eat them, but it's an off-label thing and here on BYC we try not to recommend off-label things.)
x2
Most mites are invisible to the naked eye and don't actually live on the chickens--they live on the roosts and nesting boxes. I'd thoroughly clean the coop, just in case.