Underground activities are not counted in GDP because.

ECON 211

f a decision made in the 1930s and no one has gotten around to changing it yet.

some underground activities are illegal, and the government doesn't want to place, for instance, drug dealers on an equal footing with small businesses.

there are no written records of underground activities.

although it is possible to accurately measure underground activities, it is harder than measuring above-ground activities because with the latter, one doesn't have to dig as deep.