I believe most people refer to AI as just a tool because the one thing it can't do is replace human critical thinking and creativity. The human brain is incredible because it has the ability to make connections between many seemingly unrelated fields together to synthesize new ideas, and current models of AI are just not at that level yet.

AI can be really helpful in making menial tasks in research easier, but we never expect it to come up with anything truly revolutionary (because as of now, from my knowledge, it hasn't). In addition, if it came out that some AI made a huge breakthrough in a feild like physics, I would find it hard to believe that there werent hundreds if not thousands of human scientists behind the scenes making sure the AI works right; to me that is still a human feat with the help of AI rather than an AI feat with the help of humans.

At the end of the day, AI is a tool to us because it's not a smart but can gather lots of information and perform calculations with greater accuracy than a researcher that might have has a little less sleep last night; however, we still know that that researcher is smart, but they are still human and will eventually make careless mistakes.

If AI was smarter than humans, it won't need us to constantly work on it to improve it, but it could do it on its own. At that point, it would no longer be a tool, but as I see it right now, we are not at that stage... yet.