Because they aren’t uniform or something. This was the law back in the 60s. That’s why you don’t find double yolks in store bought eggs. The farmer’s kids would candle each of them before packing to ship out. Candle meaning hold up to a light to see inside. That was their livelihood so it was an endless chore.
I know that you shouldn’t incubate double-yolked turkey eggs; but we’ve been eating double chicken eggs all my life. We find them routinely in eggs from the grocery store.