Not certified for single-pilot operation.
It’s handy to have another person up there to run the FMS, jabber on the radio, hold the pilot’s beer, and the like.
And then there is the oft told airline story about the captain giving his level off announcement over the PA: weather, arrival time, yadda, yadda, yadda.
Still keying the mike, he dropped his hand and rested it on his leg and announced to the First Officer, “Boy, I sure could use a cup of coffee and a blow job”, not knowing that the entire plane had heard the exchange.
One of the flight attendants turned to run up to the cockpit to let them know about the PA. A little old lady sitting in the last row cupped her hands around her mouth and yelled,”you forgot the coffee!!!”