Because she only navigated in the East blue for her whole life, and so she didn't no better. The system of navigation in the grand line is far different from anything she encountered on the normal sea.
Nami was born in East Blue and stayed there until she met Luffy. She never visited the Grand Line until she was a Strawhat pirate. She was robbing many pirates in the East Blue, she targeted Buggy to get his map of Grand Line so she could go there and steal from even bigger pirates (therefore, get bigger treasure)