Because nobody has specified how exactly that works. There is a current proposal to the standard committee to make it work. It also lists some of the difficulties:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2015/n4471.html
Update: Here’s the newest version of the proposal:
http://open-std.org/JTC1/SC22/WG21/docs/papers/2015/p0091r0.html