Because Berber as an ethnic designation can be used to refer to any of the following:
The people of the Moroccon Kingdoms to modern day Morocco.
The Taureg people.
The ancient Garamantes.
The ancient Libu/Meshwesh.
None of which are exactly the same thing as with the examples of India and China. (Although there is probably a direct line from the Libu to Garamantes to the Taureg, Morocco holds a huge debt to Arabian influence and starts with Idris' Kingdom of Fez).
It would similar to including the Celts as a civilization, though. It's also a much more specific designation than, god forbid this be repeated, the "Native American" civilization of Civ 4.