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It's because natives learn from hearing the vernacular spoken. Non-native learners learn a lot of that in a structured classroom form and as a result learn constructions. Different experience. 
We tend to say "should've" in speech rather than "should have." So to many people's ears it's "should of." Non-natives make other mistakes that natives are generally immune to, like misuse of indefinite vs definite articles. Since we just learn that organically we tend not to struggle with the bizarre rules around articles in English.
Notice, too, that many native speakers will say, "that's a whole 'nother ______," but usually write "that's a whole other ____." In that case, the written is correct the but the spoken is "wrong."
You have made good points. I’m Team Addictive now. >

I find grammar corrections really addicting >

Careful, that’s an addicting way to look at it >

People saying/writing "should of" instead of "should've" aggravates me so much more. >

I see more native english speakers say or write "should of" than people who's second language is english, I always wondered why.