Here's why:

* Erosion: The Grand Canyon was carved by the Colorado River, which eroded the rock over millions of years. This is a physical process where the river's force breaks down and carries away rock, but doesn't change the chemical composition of the rock itself.

* Weathering: Weathering processes like wind and rain also played a role in shaping the canyon. These processes break down rocks into smaller pieces, again without altering their chemical makeup.

While some chemical weathering (like oxidation) might have occurred, it's not the primary driver of the Grand Canyon's formation. The sheer scale and dramatic shape of the canyon are primarily due to the unrelenting force of physical erosion.