Why is little known about 90 percent of Earth's history?

Humans have only existed for around 10% of the Earth's history, therefore almost 90% of the history of Earth is either hypothesised from available evidence or otherwise unknown.

Also we learned what we know from fossils, different kinds of rock formation, and gases trapped in ice and ocean floor cores. These gases can tell us what kind of atmosphere the Earth had, What kind of plants, even the amount of snowfall for a given year.