Here's a theory: people who buy McNuggets on their own to eat as a meal for 1 to 3 people want to buy 20. The 10-piece is for combo meals, with fries and a soda

Here's a theory: people who buy McNuggets on their own to eat as a meal for 1 to 3 people want to buy 20. The 10-piece is for combo meals, with fries and a soda; it's on the menu because if they have a 10-piece McNuggets box for the combos they'll sell it on its own if you really want one, but they'd prefer not to. They want you to buy either the 20-piece, or the combo, and they don't want to give any price break on just buying 10 nuggets because that's not to be incentivized.