1. Competes against and destroys Google's primary profit center (search).
2. Google cannot copy, improve, and give away for free to use as a moat without severe collateral damage to itself.
There's very little incentive to use a search engine to find an ad ridden website to answer a question when one can ask ChatGPT directly for a much better response.
Google is rightfully terrified, Pandora's box has been opened. ChatGPT may not be the eventual sole winner of this AI text knowledge base revolution, but it's very hard to imagine a viable necessary advertising model for this technology.