The reason why is best illustrated with a bit of code:

public final class ConstructorOverridable {

  public static void main(String... aArgs){
    log("BUILD AN ANIMAL :");
    Animal animal = new Animal();
    log(" ");
    log("BUILD A FISH :");
    Animal fish = new Fish();
  }
  
  // PRIVATE
  private static void log(Object aObject){
    System.out.println(String.valueOf(aObject));
  }
  
  private static class Animal{
    Animal(){
      log("Animal ctor");
      //call an overridable method from ctor - dangerous
      perform();
    }
    void perform(){
      log("Animal perform.");
    }
  }
  
  private static class Fish extends Animal {
    Fish(){
      //every constructor has an implicit call to super(); for first line 
      super();
      log("Fish ctor. This is the earliest Tail Shape can be initialized.");
    }
    void perform(){
      //in general, this method can depend on Fish data
      log("Fish perform. Can fail since Tail Shape is : " + fTailShape);
    }
    private String fTailShape;
  }
}
 

BUILD AN ANIMAL :
Animal ctor
Animal perform.
 
BUILD A FISH :
Animal ctor
Fish perform. Can fail since Tail Shape is : null
Fish ctor. This is the earliest Tail Shape can be initialized.