It is tradition. Also in larger airlines there is often a great variation in the number of hours worked. When I retired I was an A320 captain and flying an average of 90 hours a month. I could have been a 767 captain making a higher hourly rate but would have only been flying 65 hours a month because of the trips they flew. Those numbers could change at any time based on the needs of the company. Why would you pay two people the same when one works almost 50% more hours?