Shoulder joint that is the humerus is easy to dislocate. Why?

Right hip joint

In the hip joint shown in the right, a femur enters deeply in a joint like a picture. Compared with this, a humerus does not enter deeply in a joint. It is easy to dislocate and shift. The bone which comes out of the waist is called femur. This bone has entered the hollow of the pelvis.
The right shoulder

The part of the right pelvis

What is the structure that prevents a humerus from dislocating?

Bag
The relationship between the bone head of the humeral top and the scapular hollow (glenoid fossa) are the same as the relationships of a ball and the saucer

The surround of the bone head that fitted in into the hollow is fixed with a bag so that the bone head does not slip down.

The tendon of the muscles extended on the humerus
This tendon passed along the bottom of a bag, and has adhered on a hollow.The bone head is kept from shifting from a hollow because this tendon moves.

The tendon of the four muscles
The tendons of the muscles which are between the front side of a scapula and a humerus, and the muscles which are between the backside of a scapula and a humerus are gathering. It prevents a humerus from shifting.
Supraspinatus muscle
This muscle that is one of four have adhered to the top part of the bone head. It prevents the bone head from shifting upward.
The big bag hanging on the bone head
It is the biggest one in the bags outside all joints. It hangs on supraspinatus muscle.
Ligament
Various ligaments are fixing the bone and the bone around the bone head and a hollow.

What is the role of the tendon of the muscle which extended on a humerus (tendon of the long head of the biceps)?

Like animation, even if a humerus moves to various angles, it has controlled so that the bone head does not shift from the center of a hollow.

Where are the tendons of the four muscles? Let’s look at it once again.
Four muscles (blue) shown in the picture below adhere to the bone head of the humerus.
The thing which the tendon of these four muscles was joined is called rotator cuff, and this becomes like a roof, and therefore a humerus does not shift above the shoulder.
We are looking at from the back side.
The names of muscles are, supraspinatus muscle, infraspinatus muscle, teres minor muscle.
We are looking at from belly side.

The name of the muscle is subscapularis.