Because OpenAI focuses on putting out quality models. Efficient execution of ML models is another skill set entirely. Projects like CTranslate2 (which is what faster-whisper uses) are focused on fast model execution and work across all kinds of models from speech recognition to image and speech generation and everything in between.

Also because OpenAI benefits from a certain measure of inefficiency to prevent models from being easy for the masses to run without them being in the loop extracting money as well as compiling new training data out of every inference that users feed them.