Most STT systems also tend to still train on normalized text which is free of the punctuation and capitalization complexities and other content you find in text LLMs. I suspect we continue in this way in part due to lack of large scale resources for training, and due to quality issues - Whisper being an outlier here. Anecdotally 8bit quantization of larger pre-normalized STT models seems to not suffer the same degradation you see with LLMs but I can't speak to whether that's due to this issue.