Because they are to be lifted by hand to expose the engine.
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.
Less weight equals better fuel mileage. Crampled?
Ussually headliners are made of a light weight tricot material
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.
To reduce the overall weight of the vehicle and it's centre of gravity. Carbon fibre is also stronger than steel or aluminium.
The less weight, the better the fuel mileage.
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.
Most of the late model Volvos have alloy hoods.
The materials that sarongs are made of are Rayon, Chiffon, and cotton. These materials used are all light weight .
steel light sheets that's why they rust but are light weight
Leather and other material's
There's not a huge market for light bulbs that don't let any light out.