Because if you need something to remain in the same place you box it yourself. Ruining the performance for everyone else because you don't want to handle some boxing on your own is hardly reasonable. This goes fully against the C++ mantra of if you don't use it, you don't pay for it. It's why you have sort and stable_sort rather than unstable_sort and sort.

Just because Hyrum's Law applies to an implementation of the standard doesn't mean that you should pessimise your implementation. You should actively hurt those who rely on implementation quirks