Scale is a big part of it.
Every minute, 500 hours of video are uploaded to YouTube.
Even Google can't afford to run the biggest baddest model on every single one of those hours.
I thought so, too. But they also let you create AI dubs of your videos into an arbitrary number of languages?
I also suspect that the quality of transcription depends on the popularity of the video. So maybe it's decent for videos with 000,000s of views. Same way the video codec is changed from AVC to VP9 - and occasionally AV1 nowadays.
Though they do have the money to generate terrible TTS audio for many videos. And since google can't fathom a person knowing a foreign language, use those automatically without warning.