How many people do you know who read textbooks in their spare time? And what percentage of them do the exercises in the book?

Not the original person you are replying to, but several including myself.

That said, I think that your question is a bit loaded. Textbook-level treatment of a subject is usually very shallow (depending on the subject, I suppose).

> several including myself

Nice to meet the unicorn! Meanwhile, how many textbooks are on the shelves at Barnes&Nobel? Not exactly bestsellers, right? Even the University of Washington bookstore puts the textbooks in a separate room in the basement.

> Textbook-level treatment of a subject is usually very shallow

Maybe the ones you read are. The ones Caltech uses aren't, like this one:

https://www.amazon.com/Gravitation-Charles-W-Misner/dp/06911...

Caltech has decided not to stock textbooks in its bookstore anymore, sadly (when I was there they still did)

Wow. I wonder what the rationale was.