Why Can’t I get a First-Class Upgrade?

First Class travel on all major US airlines is off more than 95% — meaning there’s always lots of First-Class empty seats on most flights to “ANYWHERE” … so why is it they go empty? Would be easy to offer “any traveler” the opportunity to purchase a FC-upgrade at check-in or at the gate — with a charge of $10-$100 depending the length of the flight. Would be easy to make them available with a special day of departure text to the airlines very frequent flyers or just about anyone with a mileage charge. Would seem like good customer service – would seem?