Why is it difficult to conduct extensive excavations at most sites important from the 6th century BCE to the 6th century CE?

People live in these areas even today.

We have seen that kings and ruling elites lived in fortified cities. Although it is difficult to conduct extensive excavations at most sites because people live in these areas even today (unlike the Harappan cities),  a wide range of artifacts have been recovered from them. These include fine pottery bowls and dishes, with a glossy finish, known as Northern Black Polished Ware, probably used by rich people, and ornaments, tools, weapons, vessels, and figurines, made of a wide range of materials – gold, silver, copper, bronze, ivory, glass, shell and terracotta.