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.
should of is probably a product of phonetic typing (those who just type the letters that match the internal audio) or when siri first launched voice typing and no one bothered to check it. Edit: Should of should’ve died a long time ago tbh. could do with a mini-crusade.
People writing of instead of 've is pretty well attested before Siri even existed so I doubt that has much to do with it