The human shoulder joint is intrinsically very unstable and relies heavily on soft tissue for stability, unlike the hip joint which has a true bony socket.

“the fact is that yeah it when we went from the ball and socket joints or the very stable simple hinge joints to these these cup and saucer type joints right specifically the shoulder which is intrinsically very unstable and just and also our opposable thumb our opposable thumb is the base of it the basal joint is terribly unstable it's just a it's we call it a biconcave saddle but it's barely that there's really no intrinsic bony stability and similar to the shoulder I mean we use an analogy that's slightly off but a golf ball on a tee it's the t is a little bigger a little bigger but that's what it is it's very flat and shallow and so we can talk about the anatomy so let's do that in in whatever way you think makes the most sense”