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.
It’s like theyre/theire/they’re - in my experience it’s mostly native speakers confusing them.
This is also a phonetic mistake. The same people also confuse “your”, “you’re”, and “yore”, because they’ve been repeating those words in a way that makes them sound the same when they do not.