Because the Earth orbits the Sun and the Moon orbits the Earth. The two are unrelated and are not at opposite ends of an imaginary axis, so it is perfectly possible to see them both.
An eclipse is where the Sun and Moon's orbits intersect from a viewpoint on Earth.
One can see the Moon during daylight hours (if weather conditions cooperate) for only the approximately two weeks a month that the Moon is on the same side of the Earth as the Sun. This means one cannot see such a phenomena during the Full Moon and New Moon cycle... By the way, here's a calendar that indicates when you can see the Moon during daylight hours: http://www.madison.k12.wi.us/planetarium/moonc al/daymoon.htm
What amazes me about an eclipse is that the sizes of the sun and the moon is perfect in coverage from the Earth. If God was a mathematician he has designed the size and distances about right.
There are times though kwicky when the moon is not quite large enough to cover the disc of the sun, this occurs when the moon is furthest from us and we are closest to the sun, known as an "Annular" eclipse. See here: