That's only an FOM rule because of a poorly worded, knee-jerk federal law, and also the knee-jerk optics of the public reaction if there was an incident and a pilot was doing anything other than staring unblinking and laser-focused at unmoving gauges for 6+ hours straight in cruise flight. Watching paint dry in other words.

Personally I'm fine with just reading, but not everyone is (some of them probably don't even know how to read anymore).