Having been involved in the WG14 discussions on this topic:

The issue is that there is a contingent of users who complains about cases where the return dynamically can't be hit but that isn't obvious statically. Consider something like this:

  int do_something(enum meow koala) {
    switch (koala) {
    case enum_val_1: return 5;
    case enum_val_2: return 3;
    /* etc., covering all the enum values */
    }
  }

Should this be required to diagnose? That's the sticking point.