AFAIU, it's just basic supply and demand. A block is mined every 10 minutes and has a fixed size, so you're paying for a slot to put your transaction in the block. And there actually is no fee listed anywhere that you pay and it guarantees you a spot, it's a bit like a stock order book where you can offer any fee that you want on your transaction and the miners will pick the highest fee transactions available to match with. $10 is about the average minimum fee size being accepted in blocks atm. Ultimately as demand to send transactions goes up, fees go up.