Because after tuple packing returns (a C return) you still need to perform a Python return. The C tuple packing function cannot return directly from the Python function.

Looking at https://github.com/python/cpython/blob/master/Python/ceval.c, here is the code that needs to be executed after the tuple is constructed:

        case TARGET(RETURN_VALUE): {
            retval = POP();
            assert(f->f_iblock == 0);
            assert(EMPTY());
            f->f_state = FRAME_RETURNED;
            f->f_stackdepth = 0;
            goto exiting;
        }

    // ...

    exiting:
    if (tstate->use_tracing) {
        if (tstate->c_tracefunc) {
            if (call_trace_protected(tstate->c_tracefunc, tstate->c_traceobj,
                                     tstate, f, PyTrace_RETURN, retval)) {
                Py_CLEAR(retval);
            }
        }
        if (tstate->c_profilefunc) {
            if (call_trace_protected(tstate->c_profilefunc, tstate->c_profileobj,
                                     tstate, f, PyTrace_RETURN, retval)) {
                Py_CLEAR(retval);
            }
        }
    }

    /* pop frame */
    exit_eval_frame:
    if (PyDTrace_FUNCTION_RETURN_ENABLED())
        dtrace_function_return(f);
    _Py_LeaveRecursiveCall(tstate);
    tstate->frame = f->f_back;

    return _Py_CheckFunctionResult(tstate, NULL, retval, __func__);

I guess you could duplicate all this code in a special "pack tuple and return from Python function" C function which you could then really tail call.