Question

why can't we move all our body parts

Answer 1

We cannot move out  all body parts as different body parts have different muscles and the body parts which don't move like our elbow backward it's because of the hinge joint and also because of the presence of involuntary muscles .

Answer 2

Bones don't work alone — they need help from the muscles and joints. Muscles pull on the joints, allowing us to move. They also help the body perform other functions so we can grow and remain strong, such as chewing food and then moving it through the digestive system.  Muscles make up half of a person's body weight.

Some types of joints and some muscles are not fashioned for voluntary movement. Hence we cannot move them.

Eg : The upper jaw