Hoods are lightweight for several reasons. They do not do much more than cover the engine, like help structurally in an accident, so they don't need to be heavy. Also, the engine area is already heavy enough without adding more weight to that area, further driving down fuel economy.
Less weight equals better fuel mileage. Crampled?
Because they are to be lifted by hand to expose the engine.
The less weight, the better the fuel mileage.
They aren't always made of light materials. For example a 57 Desota, very heavy hood. Depends on the manufacturer, the hood is usually made out of the same material as the rest of the car.
To reduce the overall weight of the vehicle and it's centre of gravity. Carbon fibre is also stronger than steel or aluminium.
It reduces the overall vehicle weight, those materials are often cheaper, and the hood itself isn't load bearing, so there's no real benefit to using heavier materials.
While they were traditionally made of steel, modern cars have many special materials. For instance, the frames of some cars are now made super-light and strong carbon fiber.
The most widely used material are various steel alloys, but there are options. Some cars may have hoods/bonnets, and even body panels out of plastics. Some are made partly of aluminium.
Well..old cars they were made out of metal
Most of the late model Volvos have alloy hoods.
You just need the right materials
Cars are made from many materials. Bodies can be made of metal, plastics, aluminum or even carbon fibers. Wheels and internal parts have many materials as well.