What excuse did Sylvester Stallone use to keep making Rocky films?

At the end of Rocky I, Rocky has a detached retina and says he is retired as doctors recommendations.
Rocky II, is believable, his eye is discussed extensively, he cannot see properly and he protects it in the fight.
In Rocky III it isn't mentioned. He says he retires halfway AND at the end.
Rocky IV he has brain damage.
Rocky V I can't even remember.
Rocky Balboa, don't even get me started, but I really enjoyed this more than the previous two.