If Callaway were to suffer an untimely death, how on Earth would WWE convince the world that The Undertaker really was dead?
It seems that his entire career has been one long, inevitable Funny Aneurysm Moment waiting to happen.
For in-kayfabe Fridge Logic; if 'Taker is some sort of supernatural being, why in the world does he spend his time wrestling?
The same reason Dark Brain forced his worst enemies into a completely harmless dodgeball tournament, for shits and giggle.
You can also interpret his character (if you so wish) as a normal guy like the rest of the roster who uses special effects and illusions to intimidate his foes into thinking he's some mythical being in the same way Batman does.
Am I the only one who loves the thought of The Undertaker being the WWE's Batman?
Considering he's done things like wrestle a match immediately after suffering 2nd degree burns from a mis-timed pyro, at least some of the mystique is real and well earned.
Some people interpret it as a sort of Who Wants to Live Forever? thing. Since Undertaker, at least in character, appears to be functionally immortal, who knows how long he's existed? Perhaps wrestling is just something he's taken up recently (as much as it can be recently to a force of nature) to amuse himself/pass the time.