I think that some people don't get eyestrain using a monitor because their eyes jump around and off of it continually throughout the day. The eye motions and strain of reading is fairly different when you read word after word, line after line for 1/2 hour or more.
An analogy would be saying your arms don't get sore picking up a book but actually you're picking up a book 100 or more times in a row. Your arm will get tired.
— I don’t get that, your mileage may vary. (The Kindle is probably still what you should get if you read a lot.)