Why the edge-triggered mode doesn't work for epoll file descriptor itself?

epoll.c

#include <stdio.h>
#include <unistd.h>
#include <sys/epoll.h>

int
main (int argc, char *argv[])
{
    int efd[2];
    struct epoll_event e;

    efd[0] = epoll_create (1);
    if (efd[0] < 0)
        return -1;

    efd[1] = epoll_create (1);
    if (efd[1] < 0)
        return -2;

    e.events = EPOLLIN | EPOLLET;
    e.data.u64 = 1;
    if (epoll_ctl (efd[0], EPOLL_CTL_ADD, efd[1], &e) < 0)
        return -3;

    e.events = EPOLLOUT | EPOLLET;
    e.data.u64 = 2;
    if (epoll_ctl (efd[1], EPOLL_CTL_ADD, 1, &e) < 0)
        return -4;

    for (;;) {
        struct epoll_event events[16];
        int nfds;

        nfds = epoll_wait (efd[0], events, 16, -1);
        printf ("nfds: %d\n", nfds);
    }

    close (efd[1]);
    close (efd[0]);

    return 0;
}