Why are the last 570 million years of Earths history divided into 3 different eras while the preceding 4 billion years compromise only one era?

Precambrian: Nothing exciting happened, just orogenic processes that basically created the planet we'd recognize today.

As opposed to...

Paleozoic with the first non-cellular life forms really taking off

Mesozoic: all the really cool stuff, like dinosaurs

Cenozoic: the planet re-cooperating from a massive meteor strike, eventually the rise of mammals & humans