The easy answer is because the FDA says so according to scientific proven studies, tests, ad infinitum all of which are available on the internet.
Just type in "egg withdrawal period for fenbendazole" or 'albendazole' or 'levamisole' or any wormer you want and you'll find different studies etc. recommending whether or not there is enough residue in the eggs to warrant a withdrawal period.
Personally I get tired of reading them and eat the eggs, have been for years and years. I'm still here typing.
That said, if you suspect that you or a family member might have a reaction to the minute residue in the eggs, by all means toss the eggs in the garbage according to the latest study/testing. Do not sell nor give eggs away nor feed them back to birds.
Here's what most chicken keepers go by regarding egg withdrawal periods:
http://www.farad.org/
http://www.usfarad.org/uploads/5/4/9/6/54966231/us_aadd_poultry_tolerances.pdf