Advertising is really lucrative. MS wants to get a bigger share of the pie, and that's done by driving people to use Bing. Also, if Bing Chat gets people used to using AI chatbots for search, that dramatically increases the cost of delivering search. Search results are not a huge chunk of Microsoft's revenue, but they are Google's main source of income. Search standardizing on LLM chatbots would take a huge chunk out of Google's margins.