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 because you’re a non native speaker. Should of is a phonetic mistake that can be traced back to repeating words you hear over and over again before you know what they actually mean