Google Search is still a large fraction of Alphabet's revenue, and Alphabet needs to be able to deliver AI results for people's searches if it wants to stay relevant in the search game.
I imagine it's much easier to have fine grained control over AI search results if you develop and own the AI yourself.
I was thinking about them fine-tuning the results for marketing purposes. But I just did a bit of research and it seems that would probably actually be illegal. So never mind, maybe?