Why Bitcoin's energy usage is directly proportional to its price

By design, bitcoin mining pays out exactly 6.25 BTC, every 10 minutes - to someone, somewhere in the world. (...)( In practice, this usually gets split between many people in a mining pool. ) The more "work" a miner does, the greater their chances of getting these bitcoins. The network automatically adjusts the mining difficulty.

If mining is too hard, it would be so unprofitable that everyone would stop mining - this would cause the mining difficulty to automatically become easier. If mining is too easy, it would be so profitable that everyone would start mining - this would cause the mining difficulty to automatically become harder.

Therefore, Bitcoin's mining difficulty always hovers around "just barely profitable for the average miner".

In other words, for every $1 worth of bitcoin mined, you can expect that it cost almost $1 of electricity and/or hardware (P.S. hardware also requires energy to manufacture).

The more that Bitcoin is valued, the more resources will be spent mining it. If (for example) Bitcoin's price doubles, so will the energy used in mining.