Why do we treat biodegradable trash the same as non-biodegradable?

As majority (about 85%) of industrial and household waste ends up on landfills, it does not matter that much whether that trash will further decompose or not. It will take certain amount of space, and stay there till the landfill is paved over... However, we are considering adding a separate categories for these two classes of trash to increase accuracy of our impact estimates.