A common line in the orthodox line of the QGD goes something like this:
[FEN ""] 1. d4 d5 2. c4 e6 3. Nf3 Nf6 4. Nc3
Here's the part that puzzles me. According to the database in 365chess Black's most common response in 4...Be7 presumably to support the knight in case of 5.Bg5 (which remarkably according to the same database is White's most common move even though after 4...Be7 no tempo is won). Anyhow, why doesn't Black simply play 4...g6 instead so he can conveniently fianchetto the King's bishop in response to 5.Bg5 ? This move does appear in the aforementioned database but way, way low. Why doesn't Black try to occupy the long diagonal with his King's bishop? Is there any strategic reason for Black in QGD for the King's bishop to remain on the a3-f8 diagonal instead?