Because they are weird, multi-jointed, spidery things at the end of your arms that have developed through evolutionary selection to be highly articulated appendages allowing you to manipulate delicate and complex mechanism and interact with the world around you.
Like, arms and legs are basically cylinders with a bit of padding on them. Hands? Hands are multiple joints per finger, the center of your hand isn't a flat, inflexible piece of bone - it's actually loads of bones and sinew and nerves and stuff, and flexing one part of it affects the shape and positioning of all of the other parts as well.
Using your own hands as reference is a good idea - they are (excuse the pun) always on hand.
.... unless of course you, like me, have hyperflexible fingers that bend in weird, spider-like shapes that just doesn't look healthy. >.<