What's special is we decided that having a different category for things humans do is useful.

It can both be true that everything humans do or create is natural because all of existence and everything in it is natural, and also that some things humans do or create are artificial or un-natural, without contradiction, because context may be taken into account, and the same words can mean different things depending on how and why they're employed.

You're asking the question backwards. We have a concept, "byproduct of human action". We needed a word for this concept. We more or less arbitrarily chose "unnatural" or "artificial" as the words for this concept.

You can argue that "unnatural" was a bad choice for this concept. But that's irrelevant to what words mean. There are other words like this - for example, "antisemitic" means "something that is against Jewish people", even though "semitic" means "of Jewish or Arabic or Phoenician etc. descent". So something discriminatory against semitic peoples is not necessarily anti-semitic.

Natural language doesn't follow strict logical rules. And of course, natural language is in fact itself an artificial, unnatural, construct.