Why is still Fee not a 1 Satothi?
Transactions are very big. So one is very expensive.

Less transactions is better for the network.

Large transactions, with old coins, do not need to pay fee. They can not be spammed with.

Any miner can choose a minimum fee. The miner chooses the math.

The commonly accepted fee is just chosen by bitcoin developers. It was their choice, no special math involved. We try to have a manageable amount of transactions per block.