Is that because NR» staff need to pay the same ticket prices as the rest of us when they travel on business? In the days of BR▸ we just used a pass or a free ticket. I remember travelling from Derby to York and back on an 'All Stations BRB‡' pass when I worked for BR.
I suspect in many cases taking a flight was cheaper, much quicker, more reliable and a more efficient use of time and doing business.
British Airways staff catch trains too I expect, and Highways Agency staff have been known to go by ferry.