Two reasons.
First, the ticket prices reflect the cost of renting the stadium and all the operational costs of staging the event. They are already charging VERY high prices to those in the better seats (well above cost) to subsidize folks like you who kevetch about $100 tickets. No one says you have to bring all the kids to sit in the cold for 3-5 hours
Second, I suspect a large part of the reason for unsold seats is simply that the unsold seats are those in the open areas of MetLife stadium where many are reluctant to commit for a New Years’ day event when the temperature could be a bit on the “cool” side.