Why is so little known about the early life of people in the Indus River and Huang Ho valleys?

I actually think that using research of course, that there werent much written records left behind of these people. Now when you do think about other civilizations for example ancient Egypt is an elongated way but very amendment and advanced. Trying to understand of these people in the Indus River and Huang Ho valleys is very opaque. Inherintly (srry if i spelled wrong) these strategies have been passed down generations by generations. However it is statened that actually there were some records left. But very amendment to read, it must have been written in acamamic language. I actually had a class on this at school today. Hopes this helps!!!

-A joyful sixth grader