We often see elliptical sky maps because of something called Mollweide projection. This is used to convert our 3D spherical measurement of the sky into a flat, 2D representation. Unfortunately the conversion from 3D to 2D is not a perfect one and sacrifices in accuracy have to be made.
With a Mollweide projection, angle and shape are sacrificed in order to preserve proportions in area. This is why they’re used to create maps of the sky and the globe.
Area is often seen as the more- important property being observed. In a perfect world we wouldn’t have to make these compromises but as we currently operate in a 2D display- dominated world, displaying 3D data can prove to be tricky.