We lack a lot information about the indus valley civilization. why is this? what kinds of information do we have about egypt and china that we don’t have for the indus valley?

We lack primary information from the Indus Valley Civilization

For example:

Indus Valley civilization: It is just a ruin, we cannot trace it to a people group that is still around
Egypt: It is still a country today, with government, state, people group that lives together etc.
China: Same as Egypt, it still lives in its native area, etc.

hope this helps