Well, I figure that making a good selling game like Mortal Kombat is a good thing to get on a resume in the gaming industry. But imagine that you put in your CV "I worked as a graphic designer in MK" and your name isn´t credited.
So it shouldn´t get confused with vanity, there are lots of legal stuff that go with it, and also respect for the people who busted their asses for years to get the project done.
So End Credits = "Proof that I was part of MK9".