#3 He doesn't need a title to main event WM:
Let's not beat around the bush, the Undertaker is one of, if not the most famous WWE Superstar of the past two decades. The man has been a prominent figure of two different eras and stayed at the top of the ladder for twenty years. His status alone allows him to headline shows. If the Undertaker doesn't have a title, he's still a draw.
For twenty four Wrestlemanias, the Deadman has been the centrepiece of every card. Some would even argue that he would not need to win the Rumble to main event WM33, as he's the main event of every Wrestlemania anyway.
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As Vince once famously said: "It's not the title that makes the man, it's the man that makes the title." Titles and title opportunities don't necessarily make stars, neither do they increase the stardom of any already established performer.