Because it isn't RLHFed to do so. It's a good example why alignment sometimes isn't a bad thing.

ChatGPT does specifically disallow medical advice and generally it is RLHFed not to provide it: https://openai.com/policies/usage-policies

What does ‘willing’ mean here? To be clear, these things are not people, or anything resembling people. They do not have will.

My friends aren’t licensed healthcare providers, but they’re more than willing to provide diagnoses.