#1 It's time

As mentioned on the previous slide, The Undertaker is 55 years old. Goldberg is 53 but has a few matches each year that are usually under three minutes. 'Taker rarely has short matches save for his bout against John Cena at WM 34 and against Styles in the gauntlet match at Super Showdown 2020.
The Deadman has been wrestling since 1987 and wrestling for the WWE since Survivor Series 1990. That's a thirty-year career. It's also unheard of. NFL athletes used to play 15 or more seasons before they were informed of the effects of the wear and tear of the game has on their bodies and minds.
Cody Rhodes' uncle is a WWE Hall of Famer. More details HERE.
Even if it is known to be scripted, professional wrestling is still a full-contact sport. The only parts that are scripted are the outcome and moves. The physical toll it can take on a performer's body is very real. The toll it can take on a wrestler that's almost seven feet tall is increased simply due to the size of the performer.
I said it before - he has absolutely nothing left to prove in the wrestling industry. He's won 7 world titles, 7 tag team titles and a Royal Rumble. Throughout his career, the Undertaker has faced high-profile names including Stone Cold Steve Austin, Hulk Hogan, Edge and Triple H. He's also battled the likes of Randy Orton, Sycho Sid, Shawn Michaels, CM Punk, Roman Reigns and Mick Foley. Not many other stars can boast that list of names as former opponents. He might still want to perform for his own reasons but perhaps it's just the right time to walk away from in-ring action following his match with AJ Styles.