As a suicidal person it can be extremely difficult to open up to another human being because my convictions of hopelessness and futility are so strong that to truly communicate them would also mean cursing the listener with knowledge that would make they themselves suicidal.
Maybe it wouldn't directly cause them to also become suicidal, but it could be a contributor. It could be one cut in a death of thousands. If you've ever seen the show Smiling Friends, the first episode is an example of how wanting to help can backfire in harmful ways.
An AI chatbot might be uniquely useful as a therapy option. I wouldn't have to feel the guilt of corrupting an innocent human being with dark suicidal thoughts. Sometimes a human connection is important for therapy, but the ability to talk things out may actually outweigh it for this context.