Because if there's one thing that the Undertaker doesn't have yet, it's a cemented legacy of good Mania performances.
I can understand if that rumor about him wanting to work with Bryan is true, but barring Rock coming back ot a match with Cena, the money with Brock is pitying him in there with Undertaker. I guess the question there is if they'll be able to convince Lesnar to go rehearse with him like they've done with all the Undertaker stuff the last few years. I'm sure another hundred grand or whatever would go a long way towards accomplishing that, since I'm assuming it wasn't part of the original deal.
Shield started with Ryback and Punk, and Kane drifts in and out of relevance every year regardless of who he teams with/is facing. This one is more personal opinion, but I find Orton just as boring and corny now as he was before the Bryan thing.
His string of good matches at Wrestlemania arguably started with his match against Batista at Mania 23. Most matches prior to that were throwaways. Unless you're going to argue that his matches against Giant Gonzalez, Jimmy Snuka, Big Boss Man, Big Show & A-train are all somehow criminally underrated, I'm not sure what you're getting at.
His match quality only got great with his match against HBK at Wrestlemania 25. The follow-ups against HBK, HHH, and Punk were all fantastic, but that's six great Wrestlemania matches in about 21 years. If you want to sarcastically take a poke at that, go for it, but all it takes is one mediocre Mania match for you to turn around proclaim that Undertaker is all washed up and Taker knows it.
At age 48, there is no one older in this industry that is still main eventing a show like Wrestlemania. If his match doesn't deliver he probably won't get another go at Wrestlemania 31, and his career ends on a dud. He doesn't want that. I don't understand what Rock / Cena has to do with anything. Could you elaborate? But his match quality, and the heat that goes with it, will probably rise to the quality of HBK / Taker if they end up doing Bryan / Taker instead of Lesnar / Taker.
Also Shield debuted against Punk and Ryback but ther first match WAS against Bryan, Kane, and Ryback. That match also got rave reviews. You can catch up on it here:
http://www.dailymotion.com/video/xw67rr_2012-tlc-the-shield-vs-team-hell-no-ryback_sport. That legitimized the group instantly. Also Kane has almost never had this degree of heat as when he and Bryan hugged it out. You're free to find us some counter-examples.
[youtube]http://www.youtube.com/watch?v=BFcXclYSCkM[/youtube]