One of my mother's pet peeves is improper grammar, most potently using the phrase "I should have went" instead of the correct phrase "I should have gone." A couple of day ago, she read me an opinion piece by someone who shared her frustration, arguing grammar mistakes (even in casual conversation) demonstrate sloppy thinking, laziness, and a disrespect for the English language. I conceded that for instances such as job interviews, this makes sense: good grammar signals intelligence and etiquette. But something about the story didn't sit well with me and I let it go.
Later, I realized bad grammar is also an example of countersignaling. When you can send multiple signals, you are best to eschew weak signals and stick with strong ones, demonstrating that you are not be confused with those who are merely adequate (as average candidates will send the weak signals in case the strong ones aren't as strong as they think). This is why you don't put that part time job from high school on your post-college resume.
Good grammar is a weak signal (with the exception of, perhaps, English professors). By making (purposely or not) common grammar mistakes, people can show they are so qualified, they don't need obsess over the nuances of the English language. This, of course, does not work with rare mistakes. "I is interested in working with you" will not get you the job.