For example the pectoralis muscle starts from sternum region and ends on the acromion or coracoid process. Why you can't say that it starts acromion process and ends on the sternum?
This is more of a convention.
The start point or origin is considered to be:
- bone which is proximal
- bone with higher mass
- bone with higher stability during muscle contraction
The end point or insertion is:
- the structure that tends to be moved by contraction
- distal end of a bone
Reference: Wikipedia - Anatomical terms of muscle