#1. He's too old
This is the biggest reason that WWE doesn't need The Undertaker. While we're being a bit liberal by saying that they don't need him for WrestleMania, the harsh reality that fans may not want to hear is that WWE doesn't need him anymore, period.
This isn't meant to be disrespectful to him, because we believe that he's one of the all-time greats. However, there's absolutely no denying the fact that he's far past his prime and has been for the last five years or so.
Each year, watching him gets more painful, as he grows older and his body goes out of shape. Even after getting his hip surgery done, his match against Triple H at Super Show-Down was painful to watch, and he looked like a complete shell of himself.
He's physically way past his prime and should have retired years ago. That's why many fans are surprised, wondering what his endgame really is.