We tend to use what we have at our understanding. Making an accurate spheroid, (which is what I assume ellipticity has to do with, since even now I still don't understand exactly what it is) that is then projected flat seems a bit beyond the scope of useful consideration. As in when drawing a map, which is flattened or in some way distorted for representation what are the practical effects of calculating ellipticity? Is there any point. Even for real world use for charts etc. would it have any practical measurable effect or point? Unless you are talking about mapping extreme cases.

If this has nothing to do with spheroids just ignore me.