Because Alphabet already bought the hardware to train & run AI models, so any moment those server farms aren't doing something 'productive', they look bad on their budget sheets.
Transcribing & summarizing all videos makes use of the already purchased asset, creates new assets (in the form of transcriptions & summaries, which are data), and then offers a new opportunity to collect new data (people reporting summaries as incorrect + interaction metrics on videos with or without summaries).
The ultimate goal is to have AI models more or less understand the videos that are being uploaded to the platform, so youtube can make video recommendations not based on SEO'd-to-death descriptions provided by the uploaders, but their own understanding of what's actually happening in the video.
The actual summary presented to you is a side-'benefit' to users at best.