Why do I need to set parameters with default settings when calling SqlPackage.exe?

I am using a SQL Server deployment project. If I declare a parameter in the project settings, I always need to specify a value for that variable when I run dacpac using SqlPackage.exe, even though I have specified a default value for the parameter in the project settings. Why is this? I unpacked the dacpac file and found that the defaults are not even preserved across parameter declarations. Does everyone have this experience? Is there a way to have a package that uses parameters with a default value where the parameter does not need to be declared for SqlPackage.exe if I just want to use the default?