I actually like my job. It's meaningful and fulfilling.
But I don't get paid by the hour, I get paid by the amount of work I do. If it's slow but I'm still required to be there, that shows that my employer doesn't respect my time and doesn't prioritize my income, and that's offensive to me.
Reading a book or dicking around on a phone goes against professional standards; even if it wouldn't endanger my position I still wouldn't do it. I want to work when I'm getting paid and get paid when I'm working, and make the most amount of money in the fewest number of hours so I can go home to be with my family.