Why is the head/neck of the femur at that 120-150 degree angle?
The answer to this great question lies in understanding the "Hip Index". I am afraid I will have to go into some technical details to explain this:



When we stand on one leg, the hip abductor muscles on that side have to work hard to keep the pelvis stable and prevent it sagging. In this case the femur is static and the hip abductors are actually working on the pelvis that tilts up or down around the hip centre of rotation. The abductors are represented as "Fm" in the diagram.

In order to do so, the hip abductors have to exert more force than the actual person's weight. This is because the length of the effective lever arm of these muscles "distance h linking H1 with the hip centre H" is shorter than the opposite lever arm "distance D between H and G representing the vertical line down from the centre of gravity".

The hip index represents the mechanical disadvantage of the abductor muscles and is around "2/1" in normal individuals. Therefor if the person's weight is 70kgs, then when they stand on one leg, the hip abductor muscles on that side are contracting at 140Kgs to stabilise the pelvis, and the person's own weight is pushing down at 70kgs from the other side, resulting in the head of femur coming under 210kgs of weight.

If the neck of femur angle becomes more straight as in your question (approaching 180 instead of the average 135), then the hip index goes up towards infinity due to the reduction in distance "h" which approches zero. This is a deformity called "Coxa Valga". If the opposite happens and the neck of femur angle becomes less straight (less than 120), then although the mechanical advantage of the abductor muscles improves, this is offset by the reduction of the actual length of the abductor muscles "distance from i to T", making the muscles weaker. This condition is called "Coxa Vara". In reality, Coxa Vara in humans is also associated with shortness of the neck of femur, which reduces the length "h" also resulting in an increase in the hip index, and an increase in the forces over the head of femur in spite of the abductor muscles becoming weaker.

Both conditions "Coxa Vara and Valga" result in the hip abductor muscles becoming less effective in stabilising the pelvis. In these conditions, the hip index can go up to 4-6/1. This means that a person weighing 70kgs could eventually end up with over 400kgs on the head of femur, leading to pain and early wear and tear. It also results in the pelvis sagging down when walking, which leads to an abnormal walking pattern called "Trandalenburg Gait".

Therefore, a neck of femur angle of about 135 degrees provides optimum conditions for the minimum possible hip index without compromising muscle length and power. Any variation results in reduced abductor muscle effectiveness, gait problems, increased hip joint wear and tear and eventually pain.