For the large part it's speculation, because very few of these companies would reveal such long term plans to the general public.

From my understanding it may be a set of several reasons for doing this, each reason feeding off one another:

• Engagement with an AI chatbot by the user generally leads to them engaging with the chatbot rather than the open web, bestowing more control and mindshare for the companies providing the chatbot interfaces at the expense of everyone else, including search engines and independent websites scraped of their content.
• Holding user data, personal or not, within that walled ecosystem allows for data profiles tailored for each user to be targeted, bundled, and sold in a more efficient and high quality way through these AI chatbots rather than through traditional browsing and tracking (one or two companies control every step of the user's browsing experience, leading to higher quality and more intricate datasets).
• Once a more "dominant" chatbot emerges as the winner of this competitive race, they will have the benefits of a massive economy of scale, being the gatekeeper of knowledge for the overwhelming majority of users seeking information (even with active dissent by the tech savvy and the privacy concious).
• Investor expectations are built upon the other reasons to be hyped for investing in this "big winner" and fearful of being left investing in the "losers", leading to opportunistic companies and providers taking advantage of of the economic environment to include these features to attract funding and indicate their willingness to "keep up with the times" to the financial world.