A new tool can be ultimately understood, and has a well-defined behaviour you can rely on. Then you can indeed become an expert in using a new tool.

But AI agent is more a new being (or person). It's not possible to understand them, as they are not meant to. Each of them has individuality, and can change tomorrow. There is no guarantee of a common behaviour.

So you could replace "tool" in your comment with "person" and you will see the flaw of your argument. No matter how many people you saw before, it's difficult to generalize the skillset (at least for humans there are some biological and cultural arguments why they can be generalized over). You can always meet someone different, who's, for whatever reason, not being helpful to you.