That's because it's pretty damn hard to do. Take my recent staff mods for example. I had to put the body parts in their new place, then I had to modify the range on the elbow joints so they'd still bend the way they should.
If you wanted to start with a rotated chest, imagine all the parts you'd have to move and rotate.
While environment objects are placed relative to the point in the middle, between tori and uke, body parts are placed relative to the starting position.
Also, there's a bug where rotating the legs makes the knees change position.
To make a long story short: starting with bent arms is relatively easy. Anything more, and it gets confusing very quickly.