Because man is its very source of life, sin is a parasite living off of the human.
Indeed, evil's parasitic lust for man is never satisfied.
But it is only theoretically true that evil can swallow everything.
Some things, when swallowed, begin to work from within and break the inner logic of the "swallower."
This can be seen in history: marauding tribes conquer (swallow) a higher civilization and are in turn swallowed up, assimilated into it.
It can happen to a person, to an idea.
When evil seems to destroy and disfigure, a process begins that has its own dynamics, in which evil is unable to absorb its victim, and the opposite takes place.
Thus when a great idea is gulped down with zest, it can work in unexpected ways and cause unanticipated changes.
For instance, the Roman Empire's destruction or swallowing up of Jewish life and ideas eventually became connected with its downfall.
In the larger perspective then, evil cannot really win because it has no substance.
The more it swallows, to survive, the more is it liable to crack at some point.
–Rabbi Adin Steinsaltz
From “Tohu and Tikun” in In the Beginning by Rabbi Adin Steinsaltz