Hi Abhijit,
You do need to define each of your constants in the yaml file. I'm not sure why exactly and I think you have a good point, since I know defaults are stored in the Mendix model. Luckily you should only need to do this setup once and then copy the configuration to other environments.
Perhaps you could organize the yaml file in such a way that it is obvious which constants are intended to change per environment, and which should be static? For exmaple, list all of your "static" constants first, then add a comment, then add the "dynamic" ones.