As far as I can tell, a lot of therapy is just good common-sense advice and a bunch of 'tricks' to get the patient to actually follow it. Basically CBT and "get the patient to think they figured out the solution themselves (develop insight)". Yes, there's some serious cases where more is required and a few (ADHD) where meds are effective; but a lot of the time the patient is just an expert at rejecting helpful advice, often because they insist they're a special case that needs special treatment.

Therapists are more valuable that advice from a random friend (for therapy at least) because they can act when triage is necessary (e.g. send in the men in white coats, or refer to something that's not just CBT) and mostly because they're really good at cutting through the bullshit without having the patient walk out.

AIs are notoriously bad at cutting through bullshit. You can always 'jailbreak' an AI, or convince it of bad ideas. It's entirely counterproductive to enable their crazy (sorry, 'maladaptive') behaviour but that's what a lot of AIs will do.

Even if someone makes a good AI, there's always a bad AI in the next tab, and people will just open up a new tab to find an AI gives them the bad advice they want, because if they wanted to listen to good advice they probably wouldn't need to see a therapist. If doctor shopping is as fast and free as opening a new tab, most mental health patients will find a bad doctor rather than listen to a good one.