It's not a complicated issue at all. They just don't feel like doing it.
The way this games models work is that it plasters one image over another. That's why your in-game character settings largely do not matter. Your characters body type will always be based on the armor you wear. Height is probably the only thing that matters, but I reckon that's done by simply stretching the image of the armor...not a big deal.
Things like breast size however do not matter. Part of the reason why the variables in character creation are so very, very limited.
With the models being like that, there is no such thing as "barefoot" in the game, because the standard shoes are considered it. This sounds obvious, but it's important to bring it up here. Why?! Because that's exactly the whole point of this.
All that needs to be done is swapping that part of the model for a barefoot texture...and voila. There is literally zero coding needed to be done (not counting the rendering process and extraction/inclusion of data done with every graphical change as 'coding' here). Only a texture swap that they do with every single piece of equipment in existence. The only difference is that it would be a texture swap of part of the base model rather than a piece of equipment. But as your example shows, they already modify it with certain footwear. If they didn't and it was fully a model swap, the legs width would have a sharp change at the point the footwear texture starts for any model different than what they take as baseline. Unless that's actually unified regardless of setting, of course...
Worst comes to worst, there is zero difference between making shoes and making "barefoot" equipment. Like there's the "Emperors New" set, having one that replaces the look with bare feet (that are not floating) would be the easiest way out (though a bad one, in my opinion).