Why do you have soulderblades?

Answer 1

to provide enough surface for the muscles to attach your arms to your body, as the only bony connections between your arms and your body, the collarbone (aka clavicular bone), is far too weak to actually hold onto it by itself.

On a related note, most four-legged mammals have no bony connection at all between their front legs and their body, it is merely held in place by the muscles that are attached to the shoulder blade.