This is nothing more than mental self loving. If the Bible said that there were 9 days you would claim that there were 9 eras. If the Bible said that there were 4 days you would claim that there are 4 eras. You are arbitrarily dividing up time to fit your preconceived conclusions.
What is stopping me from claiming that there are 20 eras?
The focus is on events, which are marked in the rocks in seven places.
There are more than 7 events marked in the rocks. If the Bible said that there were 12 creation days you would claim that there are 12 events marked in the geologic record. We can all see what is going on here.
You could just as easily divide it up into 13 sections: Hadean, Archean, Proterozoic, Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Paleogene, and Neogene. Each of these sections can be further divided up. You are just picking the number 7 because that is the number you want. There is no objective reason why geologic history should be divided up into 7 sections and no other number of sections.