"The airlines would rather have an empty seat than sell it for 50 bucks on a 200 dollar flight." Makes no sense, the marginal cost of an addittional passenger

"The airlines would rather have an empty seat than sell it for 50 bucks on a 200 dollar flight." Makes no sense, the marginal cost of an addittional passenger isn't even a bag of peanuts anymore, why wouldn't they want your money, even if its $10? Plus luggage fee. I'm sure you have a valid reason, so please share.