They cannot make it more efficient because the algorithm adapts to become less efficient. Miners design new hardware and throw out their old hardware as quickly as they can to try and stay one step ahead of their peers, but their peers catch up, the algorithm de-optimizes and the game begins again.

CPUs are able to do more with the same TDP each revision, Bitcoin can only perform the exact same 7 transactions per second no matter how much faster the underlying hardware gets.