It is because of multiple angles in between the sun, earth, and moon.

O.k., there is allways one side of the moon illuminated, and one side is allways dark. With the way the moon orbits the earth, the same side is allways illuminated, and the same side is allways dark. when we see the moon at night, we are only seeing a portion of the illuminated side. I allways find it fun to remember that the moon is a spherical object orbiting the earth, and I work out the angles in my head, and BAM, It suddenley seems like a big rock in the sky(A lot of people only see it as a flat paper disk in the sky).

When we se a cresent, only a sliver of the bright side is visible, and when we see a full moon, we get to see the whole side.

Altogether there is the new moon(nothing visible), waxing cresent, half moon, waxing gibous, full moon, waning gibous, another half, waning cresnt, then it goes to new moon again. any questions, send me a message.