...in general, almost EVERYTHING is designed to operate optimally around room temp. Computers, machinery, humans, plants, wild animals. Imagine that- built to the average / most common condition.

Beyond AC, air and fuel are a little denser at cooler temps, improving efficiency. Heat rejection is more efficient too. Around room temp everything is set up to be in the Goldilocks state- juuust right. Colder, and the engine has to use energy to stay in the good spot.