I don’t think you understand how truly traumatic it is for us grammar and spelling purists to see those mistakes in otherwise fine writing, put out for public consumption. If you are sitting in a room and someone started scraping their nails on the chalkboard, would you just sit there quietly and not say anything.
Sure, you could, by force of will, do just that. But you should acknowledge how hard that would be. How difficult it is to stand idly by while travesties of language are occurring right in front of you!
So you should show a little empathy when someone points out a grammar or spelling mistake. They are trying to help, and it’s something they are almost incapable of stopping themselves from doing.