Ha! I love your grandmother.

Sure, most people can teach themselves the art of screenwriting, or any writing, however, working with a mentor can be invaluable. I worked with one on my first script because it was quicker going that route than dinking around on my own for much longer. Life's short. Get help when you need it.

Also, there are also a lot of people who can't read written instruction well and really benefit from one-on-one help. A lot of people, like me, learn best by example and not by reading about it.

Saying anyone/everyone can learn something on their own is arrogant and ignorant.