They're not. There are several a year. Eclipses happen when the place where the earth's and moon's orbits intersect (the nodes) line up with the sun. When that happens we have a so-called “eclipse season” that lasts a bit over a month. read more
Solar eclipses are rare because even though the moon orbits the Earth once a month, it requires a very exact alignment between the Earth, sun and moon to create a solar eclipse. Even when the alignment occurs, the eclipse is limited to a small portion of Earth's surface. read more
It takes three celestial bodies — the sun, moon and Earth — to line up in the exact way at the right time to create an eclipse. read more