Not all definitions are created equal (you seemed to imply that yourself when you said "It doesn't meet your definition of the word open") and the OSI definition is widely considered the de facto standard. Of course no definition is perfect, so if you have specific issues with the OSI definition of open source, go ahead and explain them and your suggested alternative. Otherwise you are making an argument that's impossible to engage with because it's amorphous by design.

The term open-source was coined with an express purpose closely aligned with the definition maintained by the OSI. The OSI also traces its lineage to the coinage of the term; all of the coiners endorse the OSD.

Anyone can read about this here: https://opensource.org/history

But it's well-documented elsewhere, too.

The term 'open-source' is political, as much as it aimed to 'depoliticize' free software. Its definition is and always has been normative rather than merely descriptive. The term and concept exist to serve a movement, and anyone who is invested in that movement's goals is likely to be invested in promoting a historically informed understanding of both.

As for why you, specifically should care about the concept, I think I don't have the patience to make the case right now. But if you're genuinely interested in that question, I'd be happy to provide links to resources if you let me know what sorts of media you enjoy/have energy for.

Because if you disregard it you will constantly be getting in online arguments with license nerds

I care for the same reason I don't like deliberately confusing product packaging.