Let's look at it like this: a block on the blockchain has a maximum size. A transaction has a minimum size. So, each block contains a maximum number of transactions. Miners must waste a certain amount of energy hashing a block to ensure the security of the network (through the incentive mechanisms described above).
So, we can compute a practical energy/block, which translates to an energy/typical transaction.
Now, the block size is a kind of free parameter - the blockchain guarantees wouldn't be significantly different with 1GB blocks instead of 1MB. And that, in turn, would reduce the energy cost per transaction dramatically, and allow the network to process more transactions/second.
Similarly, if there were fewer people putting fewer resources into mining bitcoin, the energy cost/block could go down. This would affect the security of the network though, as it also brings down the cost of a 51% attack.
But miners are opposed to such changes, which have been proposed before. So, in practice, the bitcoin network as it is today has a huge energy cost/mined transaction.
Again, if you don't believe this, you should ask yourself what stops the bitcoin blockchain from processing 1 billion transactions/second instead of the current pitiful ~2 hundred transactions.