Why YSK?
The first person who typed “should of” probably heard of it in real life that was meant to be “should’ve”, they typed “should of” online and readers thought that it’s grammatically correct to say “should of” which is in fact wrong and it became widespread throughout the years on Reddit.
I hope something could start to change.
the reason “in some dialects of English native speakers really do say ‘should of’ etc” is phonetics.
What the author of the first link claims (and the second link explains in a more accessible way), is that it’s not just that for everyone. Like some native speakers really do say “of” sometimes, even when it’s stressed and doesn’t sound like “'ve” at all. So for them it wouldn’t just be a spelling mistake, but a different grammatical construction.
last I checked it was never added to the dictionary
Some dictionaries (e.g. Merriam-Webster) actually do list “of” as an alternate spelling of “have” (not in the sense of a genuinely different grammatical construction though).
Obviously it’s not considered standard by anyone, which is also why teachers call it incorrect, who (should) teach the standard dialects.
Language of course is living and ever changing, but the line must be drawn somewhere lest we devolve into shouting and grunts like neanderthals
Language changes whether you and I like it or not, and efforts to stop that from happening are generally unsuccessful. You can also rest assured that a simple change in what is considered correct grammar or spelling (which, as far as I know, nobody has been suggesting in this case so far, but it seems like that would be the “worst-case” scenario from your perspective) would not lead to us or language “devolving”. Also, while we don’t know anything precise about how Neanderthals spoke, most likely they sounded more or less like us and did not communicate by “shouting and grunts”.
