You can ask the same question about social media or any Big Tech product today. The answer is somewhere between convenience, a better product, and inertia ("everyone else uses it", etc.).

If Google comes up with an AI that integrates with Gmail, Docs, Drive, Search, and all their other products, and leverages the amount of information they already know about you, then they can provide a much better UX than any competitor could.

I don't doubt there will be OSS competitors that don't do any mind manipulation to get users to purchase something, but most users won't use them. Just like we have sane alternatives to all Big Tech products today, and they're mostly only used by a niche group of technical users.