People are built differently. The UK short is 21.5 inches from waist to hem and hangs down to just below my knee; to be at the knee, I would need to get an extra short, which is 20 inches. One of my brothers could go even shorter with no shame, as he has short legs and more than half of the length is in his shins. A variety of sizes means they can fit different body sizes.

And as Adam said, many women prefer the shorter lengths. A few weeks ago, I worked an event with a young woman wearing a micro UK original and fishnet stockings. She said she likes the UKs because they are heavier material that isn't as likely to fly up, and the pockets are very useful. (She got a bit annoyed when we kept point to her as an example of crossdressing, though. )

So it comes down to... why not make the shorter lengths? If you don't want to wear one for yourself, get one for your wife/girlfriend/female SO. It's all about freedom.