COST.
I wrote this summary a while back.
@nuthatched above is correct, with a Caveat. The studies were NOT interested in the long term health of the birds - only their health over their productive life (approx 22 months, total) - and whether food related losses (mortality, reduced production, etc) exceeded costs to increase the nutritional value of the feed. Historically, commercial egg production has been a very low margin business - a couple % increase in feed price could be the difference between being profitable or not. Keep in mind also, their replacement costs are MUCH lower than our own.