You used to be able to take any open seat in your cabin once the doors closed. You might move closer to the front, grab an aisle seat, or head for an empty row in the back so you could stretch out.
For any free-market capitalists out there, this is called "market manipulation." It's the same as spoiling a product and letting it go to waste rather than selling it at the market price, or giving it away, because you'd rather keep the price artificially high. It's part of the grift. They make more by price gouging in times where passengers are desperate than they lose by flying with empty seats. If they were to lower the price to the point where people would actually want to buy them, then the gouging would be too obvious.
On a recent flight I saw the emergency exit was empty so I asked if I could move there and the flight attendant said sure but it would cost like 100 bucks and I was shocked, I remember they would ask if people could sit there and then walk you through the precautions