Why does the sun shine directly at the equator at the vernal equinox and the autumnal equinox?

Because when the direct sunlight moves south from the Tropic of Cancer to the equator, it is the autumnal equinox solar term in the northern hemisphere, and when the direct sunlight moves south from the Tropic of Cancer to the equator, it is the vernal equinox season in the northern hemisphere. The vernal equinox and autumnal equinox are the same day as day and night in all parts of the earth. After the vernal equinox, the days in the northern hemisphere are long and the nights are short; after the autumnal equinox, the days in the northern hemisphere are long and the nights are short.