All they do is read a bunch of books and write essays on them, they almost never take actual grammar / grammar theory / copyediting classes, so why the heck do they think they are some kind of authority?!? Simply having an English professor (who doesn't necessarly know real grammar rules) correct your grammar mistakes in the essays does not mean that you actually learn grammar. It's naive to believe that taking one class in grammar does makes you an expert. Even if it did, most undergrad English programs don't require it.

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I took college grammar as a Math major and it was a required course for all English majors.

English majors generally are there because they have a love of the English language. They study and dissect word choice and sentence structure. Even if a grammar class is not actually required, they read so many varied types of literature, have to dissect poems, and sometimes read Old English and in the process learn the foundations of our language, that by the time they graduate, English majors have a pretty good idea about how to properly construct a sentence. Correcting people is the only time they get to show off this knowledge, so some abuse the power.