Why can't you beat time?

Mortality / immortality.

Time: The Space-Time opposition often associated with Shem and Shaun, also a reference back to the Mookse and Gripes fable which was introduced as an illustation of the "space time problem"

Beat: to conquer time (immortality) and also to beat time in a rhythmic sense.

Alice's Adventures in Wonderland:
"I dare say you never even spoke to Time! [The Mad Hatter said]. "Perhaps not," Alice cautiously replied; "but i know I have to beat time when I learn music." "Ah! That accounts for it." said the Hatter. "He won't stand beating."