Why are the Archean and Proterozoic eras not divided into periods?

The history of planet Earth is usually divided into zones, epochs and periods, which differ from each other in certain features. The main unit is an era, each of which has characteristic fossils that have come down to us. The era of Archaea is not usually divided into periods, since the boundaries between periods are considered to be the global extinction of some organisms, which were then replaced by others. This is not the case in Archea. But already in the next era, the Paleoproterozoic, there were 4 extinctions, and it is divided into 4 periods.