i guess they should all be on the same plane, at the same height. if its stacked two layers high, the eggs on the top may get different temps or humidity compared to these on the bottom, and the ones right in the middle get less air, and so on. or, what if one of the eggs on top goes bad, it could ruin the eggs beneath. what if one egg starts to smell nasty and it needs to be removed.
but i think it all comes down to: its just more practical and most people do it that way. its just the obvious thing to do, just like how you would put a batch of small breads in the oven.
EDIT: i cannot resist....
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