That's like saying the evolutionary tree of life is just a set of arbitrary rules someone came up with. It's not, it's a description of an existing evolved system.
Grammar wasn't handed down from on high by an authority, it's an evolved, undirected cultural artifact. There were definitely influential actors, like the (IIRC) monks who first developed the capitals, lowercase, and italics we now use without a second thought, or Noah Webster's personal preferences for spelling which he published and we now largely accept.
> To me, it says, "I went to a good school system and was taught correctly!"
I'm not sure. I guess it depends on the test - if it's asking "which is the past participle form of 'run'", then I'd agree, that's just memorization. But if it's simply asking "which sentence is more correct" and one of the sentences has an improperly conjugated verb, that's not something one even needs to be formally taught if they've read much.
If you can't string a sentence together such that it can be read unambiguously, how can I expect your code to be readable?
> I've met more people who couldn't figure out their and there or you're and your to save their lives who were both fantastically good software engineers and effective communicators.
I'd be willing to bet if you laid out two identical sentences, one with "your" and one with "you're", these people you speak of would be able to choose the correct one. They may not always use the correct one in the quick, ad-hoc process of writing a sentence, but I'm sure they know right from wrong if asked to scrutinize.