Sumer and the writing advent

Sumerian is the oldest written language in the world.
It was spoken in Mesopotamia since 4000 BCE.
The earliest writing that were found, are dated to 3300 BCE : they are some clay tablets with cuneiform writing. The subject were about business exchange ... I guess that administrative documents should already be numerous ... 
I am very surprised because the old language had a sophisticated grammar
Enough sophisticated to write the famous story of Gilgamesh, a legend that had influenced a lot of civilization.

Reference :
Article about cuneiforrm writing (in French...)
About Gilgamesh