Why does curriculum basically start from determining the learning objectives?

You cannot plan a scheme of learning without first identifying the learning objectives.

The scheme of learning is a plan of how you will deliver the course so that each student will meet those objectives.

The summative assessment at the end (or over the duration) of the course will test that those objectives have been met so only deserving students will be awarded the appropriate qualification or recognition.

It all hinges on knowing and understanding what the learning objectives were in the first place. :)