4x4 truck wheels have to bear a lot of force from the weight of the truck, especially when starting and stopping. Weight is multiplied by speed to calculate force and trucks are heavy before they are loaded and even heavier when in use. This makes them generate a huge amount of force that has to be supported by not only the frame, suspension and axels but especially the wheels.
For very heavy loads, magnesium alloy 4x4 truck wheels are used, but not because they are lighter. Magnesium is used because it is stronger than steel. Racing wheels made from magnesium can be much thinner than steel which actually lowers the weight. Magnesium truck rims are just as thick and similarly heavy as steel wheels, but much stronger.
Since 4x4 truck wheels have to support a huge amount of force they are often made from steel which is very strong, readily available and compared to aluminum, magnesium and composite wheels very affordable. This increases the weight of the wheel but generally does not affect the handling of the truck, which is already heavy. There are also many more bolts on these rims because of the rotational force that is applied to the wheel.