PKing and sparring both require a different skillset, one not more "difficult" than the other. PKers need to be able to spot out 0.5's and get there unharmed, stealing other peoples' kills, etc. Sparring requires players to anticipate the enemy's movement, think quickly, and be unpredictable.
It's sort of like saying Halo is harder to be good at than Call of Duty. Actually... Never mind.