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Why are Precambrian rocks difficult to divide into small geologic time intervals?

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A detailed geologic time scale chart displaying Earth's history from the Hadean Eon over 4.5 billion years ago to the present, organized by eons, eras, periods, epochs, and ages with color-coded columns and corresponding dates in millions of years. — Precambrian The stratigraphic chart of geologic time.

Precambrian rocks are difficult to divide into small geologic time intervals primarily due to a significant paucity of fossil remains.12

In the subsequent Phanerozoic Eon, geologists successfully subdivide and correlate strata by utilizing the abundant fossil record of life-forms that evolved over time. Because Precambrian sediments lack these widespread, evolutionarily distinct fossils, it is not possible to create fine-scale subdivisions such as epochs and ages using traditional biostratigraphy. While stromatolites do exist in Precambrian records and have been used for some limited stratigraphic division, their application remains restricted.132

Consequently, the subdivision of Precambrian time relies heavily on chronometric scales and isotopic age determinations. Furthermore, rocks within Proterozoic orogenic belts are often too deformed to allow for reliable correlation between different regions, further complicating the establishment of a detailed, global geologic timeline.12