Having got a tutor for one subject for one of my children, I'd say that tutors are about motivation/structure/instilling confidence not just subject matter. Our tutor has been great as a general mentor during the period leading up to exams, not just as a subject expert.

I have experimented a bit with AI and have found that AI isn't always great at catering precisely to the syllabus/marking criteria of a specific exam boards even when you give them all the docs publicly available (although I haven't tried frontier models). Somebody who has worked as an examiner for the exam board knows the tricks for how to structure longer answers to both get the marks and to not waste time in the exam. Of course, it may be that I haven't prompted well enough, but if I can't manage to do that, then I probably can't expect my children to!