to those of you who get bored at work if there's lots of downtime, why?

I read posts about people quitting jobs because they're boring or there is not much to do and I don't get it: what's wrong with being paid for doing nothing or not much at all?

Examples I can think of: being paid to be present but only working 30 minutes to 2 hours every 8 hours, or a job where you have to work 5 minutes every 30 minutes.

What's wrong with reading a book, writing poetry or a novel, exercising, playing with the smartphone... and going home to enjoy your hobbies fully rested?

Am I missing something?