Of course, the Earth is not really an oblate spheroid, either. That's just a slightly more accurate approximation than a sphere. A quick look out the window reveals that the Earth is bumpy, covered in valleys, hills and mountains that are not included in the ellipsoid description.

As they say, "all models are wrong, but some are useful." The trick is to choose the simplest model that works. The right one to use depends on what you're using it for.