There are 4 spell lists right now that are built from 2 essences.
For example
Arcane = material and mental
Divine = spiritual and vital
So why don't we have essence list?
So if you play a wizard it will tell you that you get access to material and mental. This would remove the need to type out all the different spell lists. Just the 4 even if you expand the spell system with a spell list that does not exist right now.
There could also be essence specific classes/archetypes. Without having to add a new spell list.
Because some spells appear in more than two lists, and some are exclusive to one.
That's not how it works. There is no direct correspondance between Essence and spell, it's their combination that gives you the magic (or your approach to it).