Why all java services created are final and static in esb folder or java class created for java or flow service?

Hope this helps you understand.

A Java service is a public static method in a Java class file on Integration Server.

Because Java class names cannot contain the “.” character, services that reside in nested folders are implemented in a class that is scoped within a Java package. For example,
a service named recording.accounts:createAccount is made up of a Java method called createAccount in a Java class called accounts within the recording package.

For performance reasons, it is recommended that you make the Java class final. Integration Server does not create an object to save instance data. All methods of the class are static.

For more details, refer the service development documentation.