Because after the constructor finishes, the object is created and then you have no chance of changing a final variable reference.
Compare this to a real world example.
Your gender is final. You cannot change it after you are born. Most cases you can find the gender before birth and in some cases you know the gender during birth. But after birth thats it, you are either male or female and you have to live with what you are, no change
Well ... these days there are surgical procedures to change gender but I am not even going to go there