Would it make too much sense to build the same ski in a continuum of sizes from very short for children up to something that would handle the biggest and burliest adult skier?
It's a complete PITA to scale flex characteristics especially in natural materials like wood. The exact same build, if right in the 176-184cm sizes, would be waaay too stiff in the short sizes. So you need to graduate changes, including thickness and composition, as you scale. That's added ski cost right there.
It's also a complete PITA to keep the same or even similar turn radius/ taper characteristics as you scale overall length. The sidecut in the shortest lengths fairly must be completely different than in the longest.
So ... a completely different ski in various sizes, carrying the same numeric designation. You see where this gets confusing?
There wouldn't be men's, women's and children's designations. Everyone could be fitted to the ski that best suits their height and weight and skiing style. It wouldn't be hard to imagine a family where everyone is on the same ski, but Mom is on the longest ones, the teens in the middle and Dad is on the shortest ones.
I sense yet more confusion.
There is considerable overlap in sizes between adults and kids and between men and women. The special designations of men's, women's and kid's skis seems more of a marketing gimmick than a practical solution.
Maybe so, but engineers love them because they specify the direction one can err in. E.g. a slightly too stiff flex in a unisex downscaled ski is something one can get away with - but not in a womens' or junior ski.