If the moon is made to light the sky why isn't it always full?

Full moons are great, I love them because I can see where I'm going without a torch. But sometimes there's no moon or just a crescent and it's not as bright, sometimes really really dark. So why god not make moon full all the time?
If the moon is made to light the sky why isnt it always full?
If the moon is made to light the sky why isn't it always full?
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