Null as a default is useful / a big simplification when it comes to reflection/meta-programming.

Consider this piece of pseudo-Java/Spring code and think how you would do it if the platform you were using forced you to either declare a and b as optional (which they are not) or assign them some value between the IoC container instantiates the class X and wires the values for a and b.

   public class X {
       @Autowired
       ServiceA a;

       @Autowired
       ServiceB b;

       @PostConstruct
       void init() {
           // a and b are instantiated by the IoC container and
           // are fully valid for the rest of the execution
       }
   }