I need an ice pack.
I need an ice pack.
Left-handedness is slightly more common (and less detailed) than precise adjustments to body proportions.
That said, it would still be more data that needs to be stored and transmitted about our character to every other character, so it can't just be added without adding to the load of data transfer – so if they ever did add the option, it would be theoretically "instead" of something else they could do with the same data.
As much as I like having the option to pick handedness on characters (being left-handed myself) I would prefer to get things like more face customisation options or gear dye channels if they ever added more character data.
There's an important distinction here though.
As much as buttsliders are a meme, people have been asking for that and other character customisation pretty much since launch. We're not getting it likely because it's almost certainly going to require a full extra data array added to our characters, if SE were willing to do that, the data space would likely be far better off being used as a true glamour log. The current plate system suggests that they are not.
By comparison, left handedness is a simple binary switch and I'd be very surprised if there wasn't room to stuff it in an existing array.
By your logic SE shouldn't have added the Regalia because people would started complaining that it doesn't have realistic driving physics?