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".