I love to read, but I am tired to see most of the time the preposition IN put on the sentence: She sat in the chair. If the book or anything else would be where the person sat before, the authors would always used ON. If a sofa is used instead of a chair, it would be always written with the preposition ON.
I don't know why they do not see the difference.

In this image, the remote control is on the chair. A person sitting in the chair would be in the chair. On portrays the idea of on a flat surface (ie not enclosed by surrounding walls/sides); whereas, in portrays the idea of in an enclosed space. This is very general. Each noun will attract its own preposition. This happens in other languages too. For example:

in a field

on a football pitch

So, I am afraid there is no clear ruling.

I have to disagree with you.
It does not make any sense when the same writer would write in one sentence: She sat in the chair, and on another sentence: She sat on the sofa.
At a library I found a book with instructions for those who wants to write a book. The first thing it says: Do not write: She sat IN the chair. To sit in the chair, someone has to brake the upholstery or wood. I also found in the dictionary: Sitting on the chair.
On one thing you will agree with me: Not much grammar is taught in American schools. Therefore, so much confusions and not much knowledge.

Wow, that must a very comprehensive book!

I have no issue with saying 'she sat in a chair'. For me, it depicts a compfy chair with high sides and back. Sitting 'on a chair' depicts a chair with a flatter profile, like a sofa possibly.

It is said that those learning a foreign language will struggle to understand the second meaning of the language, and I think that this is a classic example of that. 'In' and 'on' a field are also both feasible with slightly different connotations. It's a tricky subject, and I would guard against pronouncing different versions as errors. Judging by its first piece of advice, I think I would throw that book out if I were you.