In my time I have come across a few instructors who were probably scared of flying and would have much preferred to do ground school only, if it paid.
In the UK casual ground school seems to be free, try it in the States, you are charged and everyone seems ok with that.
Wife of one of our students used to test her husband on the ground subjects, he struggled. She became interested in learning to fly and pretty much got 100% in all subjects and a better flyer to boot. She would probably have been good conducting ground school.