The written form is derived from the spoken form and not the other way around. This is easy for highly literate people in writing-focused societies to forget, a

The written form is derived from the spoken form and not the other way around. This is easy for highly literate people in writing-focused societies to forget, and our education tends to not explicitly address it. But there it is. To/too, their/they're, it's/its, should of/should've etc are all transcription errors, not a failure to understand or deploy standard grammar.