Because what are you going to hit up there, cumulogranite? There's a whole industry devoted to "let's keep the planes out of each other's way; luckily the sky is huge".

That violates the sterile cockpit rule and is a HUGE no no. Pilots can only read a book or talk to cabin crew if the control of the aircraft is with the other human pilot in the cockpit. Someone has to be ready to take control of the aircraft at a moment's notice in the case of a sensor, software, or mechanical failure.

> That violates the sterile cockpit rule and is a HUGE no no. Pilots can only read a book or talk to cabin crew if the control of the aircraft is with the other human pilot in the cockpit.

Sterile cockpit does not seem to apply at cruising altitude though, see https://en.wikipedia.org/wiki/Sterile_Cockpit_Rule: "The Sterile Cockpit Rule is a Federal Aviation Administration (FAA) regulation stating that during critical phases of flight (normally below 10,000 feet (3,050 m)), only activities required for the safe operation of the aircraft may be carried out, and all non-essential activities in the cockpit are forbidden."