Because the distortion induced at a particular latitude applies to everything at that latitude simultaneously. Showing the distortion only on the sea and not the land would make the map even more inaccurate and out of wack.
Every method of projection from 3D to 2D involves compromise. Different projections compromise in different aspects; each having their own strengths and weaknesses.
Add size to the waters, not the land.
Think about it. Why not add water on the google maps, and not adding land.
Because the distortion induced at a particular latitude applies to everything at that latitude simultaneously. Showing the distortion only on the sea and not the land would make the map even more inaccurate and out of wack.
Every method of projection from 3D to 2D involves compromise. Different projections compromise in different aspects; each having their own strengths and weaknesses.