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.
I’m sorry but it doesn’t fully work here. ‘of’ phonetically should not be spelled with a ‘f’, so they are already using a word that is not pronounced as it is written, might as well use “would’ve”, which removes the part that isn’t pronounced as it was traditionally “ha-”, but at least it’s still correct.
They use ‘of’ because they don’t understand (or pay attention to) the grammar of what they’re saying.