The perfect eclipse is a coincidence of this era

The Sun is about 400 times wider than the Moon and about 400 times farther away, so their discs nearly match. Total solar eclipses are a temporary accident.

The Sun's diameter is about 1.39 million kilometres. The Moon's is about 3,475. The ratio is near 400. The distances are in about the same ratio, so the angular sizes are both about half a degree. Some months the Moon is a little too far, and the eclipse is annular: a ring of Sun remains. Some months it is close enough to cover the disc completely, and the corona becomes visible.

Tidal recession is carrying the Moon outward at about 3.8 centimetres a year. Angular size shrinks as distance grows. In on the order of 600 million years, the Moon will be too far to cover the Sun even at its closest, and totality will stop. We happen to be here while the trick still works.