Time was, the postal service delivered letters a few times a day, and, say, a man could write a note to his wife to let her know he'd be home late or early, and she could write back and ask him to remember to bring something or other home with him. Then came the telephone, and all little, spontaneous communications like that started to be spoken. This is too bad, because people become better writers the more they write. If you're just writing a casual email to a pal, it needn't be literature. Writing email can be good practice in writing in a conversational way.
Many people find the subject of grammar quite intimidating.
I think they must have been taught in an unfortunate manner. Most people use perfectly good grammar the great majority of the time, and still many of them are intimidated. They may not know the names for all the parts of speech, or be able to explain how the parts are allowed to go together, but their speech and writing reflect knowledge of what is correct, all the same. Grammar is not supposed to be some arcane cult with a punitive intent. Grammar is not supposed to be some arcane cult with a punitive intent.