Why do clavicle bones break so easy?

Clavicle bones break because they have evolved to.  Sounds
strange, I know, but they serve as a disconnect for impact trauma which would otherwise
travel all the way into the neck, potentially breaking it. 

When a person falls and catches themself on their arm, the
force from the impact goes up the arm and into the shoulder.  If the clavicle bone were
thicker, it would simply pass the shockwave along its length and into the neck.  Necks
(obviously) do not respond well to trauma, so instead the clavicle breaks and the force
is dispersed into the surrounding tissues.  Certainly a painful adaptation, but one
which, like a lizard's ability to tear free from its tail if the tail is caught, was
evolved to protect the more important parts of the body.