To expand on this, it’s the naming convention that Java defines.
Package names are written in all lower case to avoid conflict with the names of classes or interfaces.
Companies use their reversed Internet domain name to begin their package names—for example, com.example.mypackage for a package named mypackage created by a programmer at example.com.
https://docs.oracle.com/javase/tutorial/java/package/namingpkgs.html