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All refractive lenses are approximations of the ideal.  This is particularly the case of single-element lenses such as in eyeglasses.

It so happens that the centers of such lenses are closer approximations to the ideal than points further from the center.  Put another way, it's easier to make good lens elements when the size is small compared to the curvature.  Not only do the non-ideal material properties cause more trouble at the edges, but the purely spherical approximation becomes less correct too.  On the other hand, you want lenses to be big enough so that you see thru them instead of around them.

The optimal shape is therefore round, as it gives the most see-thru area with the least artifacts.  This minimizes the maximum distance from center for any given overall area.  It is also an easy shape to manufacture.  At least one such manufacturing operation I knew of years ago made all lenses round, then ground down the edges to fit the particular eyeglass frames.

Another problem with large distance from center is the thickness of the lens.  Lenses to correct myopia are concave, so thinner in the center and thicker at the edges.  Some minimum thickness is necessary for mechanical reasons, so the further you get from center the thicker the element is.  This can make mounting more difficult and increase weight and the thickness of the original blank that must be ground down to make the lens element.  This is why eyeglasses with large corrections use a material with a higher index of refraction.  That material requires less curvature for the same effect, but comes with other less desirable characteristics too. 

Glasses for close use, like reading glasses are convex.  Some minimum width is required at the edge for strength and mounting.  The further that is from the center, the thicker the center ends up as a result.  That causes yet another tradeoff with wanting the inside surface to be curved away from your eye.