Because in my limited experience (I am fluent in only two other languages apart from English), "native grammar mistakes" only happen to native English speakers.
For example, I know Italian and French, yet I cannot think of any weird misspelling only native Italian or French speakers do. I always wondered if it's because of education or how grammar is taught in Anglosaxon countries that is ultimately the root cause of these errors. It is a peculiar phenomenon.
I'm French. I can't list them off the top of my head right this moment, but there definitely are annoying errors that natives do that are in the same category as "could of".
For example, I know Italian and French, yet I cannot think of any weird misspelling only native Italian or French speakers do. I always wondered if it's because of education or how grammar is taught in Anglosaxon countries that is ultimately the root cause of these errors. It is a peculiar phenomenon.