Things also get weird with air cargo. My pilot buddy says they fill the cargo area and aircraft to the weight or volume limit pretty much every flight he takes. He may fly for a weird carrier or on weird routes, but it sounds believable that there is infinite supply of "I wanna get there faster" cargo. You might see ten empty seats but merely means there's a one ton MRI thingie that's on emergency shipment in the cargo area. He mentioned they're really into containerization and a small container alone costs $200 so I am almost scared to ask how much it costs to actually ship it. Then again everything aerospace rated costs a hundred times more, so a $2 isn't so ridiculous.

Aacargo.com division of american airlines proudly advertises they ship 100 million pounds of cargo per week, and also advertise they have 6000 flights per day so thats averaging about a ton of cargo on every flight and a ton represents maybe ten empty passenger seats, so your average flight should have ten empty seats. Of course reality is the seats might be near empty at midnight and 100% full during the day, etc.