Why is every person obliged to follow moral law?
Because the outcome otherwise is devastation and death.
Simply.....that is being human. There is no way to explain how the soul and its morality works, but being of natural humanity even if one does not fully realize it, they follow and inner moral code or law.
You're not obligated, you have free will.
Many dont. We refer to these folks as criminals or much worse.
God is the true source of good and His objective moral law, which God established only for the well-being of His creation.
God created moral laws if you don't follow them you are a sinner
We're not obligated to. Other animals don't and they do perfectly fine. But, nonetheless we shouldn't do things like kill, steal, etc.