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.
Just read the second (or the first, but that is more technical) link I shared. Some native speakers do in fact seem to say “should of” even when the “of” is stressed, so in their dialect it would be grammatical.