Because iron must be separated from other materials, recycled, handled. A magnet allows you to do these things with simplicity, speed, efficiency, safety.
Actually, using a magnet you can collect only steel parts and leave other materials on the ground.
You can collect iron parts, even with an irregular shape, much faster and more safely than by hand.
And you can collect small iron parts that you cannot lift with other type of attachments.
Finally, you can lift objects regardless of their shape. (a magnet can attract what a clamp cannot get)