The proof-of-work mechanism for blockchains is broken into two elements. There's the inflation element, which is a reward for miners that is independent of transaction volume, and then there's the transaction fee element.
The inflation element exists to secure the blockchain, and is proportional to the total amount of value being held on the blockchain. As the coin price goes up (independent of any transaction activity), the amount of reward for PoW mining also goes up. It doesn't make sense to count this part of the PoW reward a cost of transacting because the purpose is unrelated to transacting and also if transacting completely halts the expense does not go away.
The inflation element exists to secure the blockchain, and is proportional to the total amount of value being held on the blockchain. As the coin price goes up (independent of any transaction activity), the amount of reward for PoW mining also goes up. It doesn't make sense to count this part of the PoW reward a cost of transacting because the purpose is unrelated to transacting and also if transacting completely halts the expense does not go away.