The computing power used for proof-of-work is not proportional to the number of transactions. Computationally, it's relatively cheap to verify the entire transaction history.

The real energy consumption comes from miners who use a crazy amount of computer power to secure the network. Bitcoin's transaction history is immutable because changing it would require an even crazier amount of computing power - too much for any malicious actor who tries to attack the network.

The amount of energy used is a feature, not a bug. There are no alternative designs known that could replace proof-of-work while keeping its security assurances, permissionless participation, fair distribution of coins, unforgeability, etc.