Does anyone else feel extremely guilty at work when they have down time?

Managers are always saying “don’t sit on your phone!” Or “I need you standing”. Why is this? It seems so irrational that I should be doing mindless tasks that don’t need doing just so I’m not sitting down. I feel like it’s been drilled into my head to the point where I feel guilty taking a break! I don’t know I just needed to vent.