But your shoulder doesn't move like your forearm, does it? It only rotates in place, with a limited range. It can't completely flip like your forearm. Your muscles lock it in place, because they have to when it's in a ball and socket configuration, because the muscles are how you move the limb around in the socket, and there are a lot of insertion and origin points near or around that socket, limiting the range of movement there.

If you want to be able to completely flip the limb, you have to make it so that the parts can move against each other more freely along with the muscles, which a criss cross configuration like this allows for, because the part that moves the most, aka the part where it criss crosses, is where there are less insertion and origin points. That way the positioning of the muscles doesn't inhibit the movement.