Why do compilers give a warning about returning a reference to a local stack variable if it is undefined behaviour?

Question

The C++ standard states that returning reference to a local variable (on the stack) is undefined behaviour, so why do many (if not all) of the current compilers only

Answer 1

It's pretty much super-bad practice to rely on this, but I do believe that in many cases (and that's never a good wager), that memory reference would still be valid if no functions are called between the time foo() returns and the time the calling function uses its return value. In that case, that area of the stack would not have an opportunity to get overwritten.

In C and C++ you can choose to access arbitrary sections of memory anyway (within the process's memory space, of course) via pointer arithmetic, so why not allow the possibility of constructing a reference to wherever one so chooses?