I was asked this a couple of times recently from students, thought i would share my thoughts here with you guys n girls.
My answer consists of 2 things:
1. Because 2d helps to rapidly build a visual library of self-knowledge in your brain, which ultimately through focus and repetition translates into 'artistic instinct'.
2. Because 2d is much faster to iterate with, enabling more rapid development of 'artistic instinct'.
Have you ever noticed when watching a master artist at work, that the subject seems to come alive so quickly, or that meaningful lines, shapes or silhouettes jump out right away, and then think:
'They seem to know exactly what shapes to put where, how to nail the composition so quickly, how to draw everything perfectly, without even thinking about it!!'
This is artistic instinct.
What you are watching is an artist call upon their whole lives worth of visual library development and self-knowledge, expressed using their 'artistic instinct' in the form of the subject.