Because they don't know how to wallwalk so good.
Even when they do start wallwalking, it's in straight lines, which will be shot down almost just as fast. Gotta keep moving fast, turning and moving forward in random directions, not just strafing on the wall.
Also, they jump too much. I always laugh at the dretches that jump at you, hit a wall and lose all forward movement and keep jumping while they slowly slide along the wall.