well, turns out there are a lot of symmetrical skis. my park skis (nordica ace of spades) and my pow skis (surface new lifes) are both symmetrical and mounted dead center.
but to address your point, much of it has to do with tradition when it comes to ski shape. make a uniform radius, have the thinnest point behind center, therefore the tail is thinner than the tip. with a center mounted ski it only makes sense that since the narrowest part of the ski is centered the tip and tail would be symmetrical, but many skis still favor improved forward handling over improved switch handling and therefore a non symmetrical shape is ideal, even if it's just barely off and the mounting point is only 1cm back. the thought process behind that is that no matter how much you ski switch, you're probably still going forward the majority of the time, so why not have the best handling properties while doing so.
hopefully that made sense
