Here's why:

* Endochondral ossification is a process where a cartilage model is replaced by bone. This is the primary method for forming long bones (like arms and legs), vertebrae, and ribs.

* Intramembranous ossification is the other method of bone formation. It occurs directly within a membrane of connective tissue. This method is responsible for forming flat bones like the skull, clavicle, and some facial bones.

While intramembranous ossification is important, endochondral ossification is responsible for the formation of the majority of bones in the human body.