#3 Nothing left to prove
The Undertaker has accomplished pretty much everything there is to inside a WWE ring, and while there are probably a few dream matches fans are still clamoring for, Undertaker just has nothing left to prove. Beyond that, if there is nothing left to prove, what is the point of him risking further injury to try to prove himself once again?
Think about it! The Undertaker is a former tag team champion, Intercontinental Champion, United States Champion, WWE Champion, and Hardcore Champion. He has main evented countless pay per views and even WrestleMania on a couple of occasions. Finally, he has taken part in some of the most breathtaking moments in WWE history.
If nothing else, there is nothing left to prove and no reason left to even prove it in the first place. Undertaker doesn't need another victory or even another appearance for that matter. He has given the WWE Universe everything he has for the last two decades and he shouldn't have to give more.