TL;DR:A lot of reasons, which you will figure out very quickly from using AI to solve your problems/answer your questions.
Accuracy
AI is very often completely making things up, spreading completely wrong information, exaggeratting correct information, or otherwise not giving you a correct and working answer. Yes, people do that too, but here we have lots of people who know what they are doing and can point out if the answer is wrong.
Bias
If I ask AI something, it will change its answer heavily from very subtle biases in the question, in a way that a human will not. This isn't great, since the goal is to get an answer that works/answers your question, not one echoing your opinion.
Sourcing
A human can say where they got information from, and that can be verified to be true. An AI really can't (it will often say it can and then give non-extant sources, broken links, etc.)
Not good for Q&A format
AI generally responds with essays, not really answers like a human does here. This makes sense, as it is after all a chatbot. A chatbot is meant to chat, not to answer complex questions, and as such does it worse.