Because the coordinates -180°,-90° in WGS84 are in angular units, and they need to be mapped to cartesian coordinates. In this case, that's done using the trivial equirectangular projection, which simply exchanges the angular units with linear units, but that process is still a projection.
I think that a related visual example of an image referenced in Geographic Coordinates is illustrated below. Perhaps my guess is completely wrong. I would like to understand what causes the "skewing" of pixels, which happens in both OpenEV as well as in ArcGIS (using data from the same satellite acquisition, packed in an NTF file). So, the question is What is this skewing effect?
A visual example
A small fragment from a WorldView-2 image, purely in Geographic Coordinates, for which gdalinfo reports the following
Coordinate System is `' GCP Projection = GEOGCS["WGS 84", DATUM["WGS_1984", SPHEROID["WGS 84",6378137,298.257223563, AUTHORITY["EPSG","7030"]], TOWGS84[0,0,0,0,0,0,0], AUTHORITY["EPSG","6326"]], PRIMEM["Greenwich",0, AUTHORITY["EPSG","8901"]], UNIT["degree",0.0174532925199433, AUTHORITY["EPSG","9108"]], AUTHORITY["EPSG","4326"]] GCP[ 0]: Id=UpperLeft, Info= (0.5,0.5) -> (-16.8511111111111,13.0025,0) GCP[ 1]: Id=UpperRight, Info= (9215.5,0.5) -> (-16.6588888888889,13.03,0) GCP[ 2]: Id=LowerRight, Info= (9215.5,7167.5) -> (-16.6588888888889,12.8911111111111,0) GCP[ 3]: Id=LowerLeft, Info= (0.5,7167.5) -> (-16.8511111111111,12.8641666666667,0)
when displayed in OpenEV as Non-Georeferenced (squared pixels, term derived from OpenEV's related Viewer icon/button)
and Georeferenced (rhombus-shaped pixels!)
As explained in OpenEV's manual, for the Georeferenced Display icon/button: Non-Georeferenced means that the view is being displayed in raw pixel/line coordinates for the source raster and Georeferenced means that the view is trying to display in georeferenced units (if any are available).
The same skewing effect appears also in ArcGIS when viewing another fragment from the same acqusition (credits for the screenshot to a friend):