This isn't specific to ultralights, but more generic to airplanes.
They could, but ailerons don't have the torque to change the direction of the aircraft like the elevators would. The center of pitch change would be a line that goes through the aircraft similar to the position of the wings. The elevators being further away from the center of the aircraft can push more. Designers purposely only have the elevators move for this reason.
The ailerons move in opposite directions when the move to induce roll. The center of roll would be roughly the center of the fuselage from head to tail. The ailerons are positioned away from this point to increase their torque.
I hope this helps.