Since the claim “there are no knowable truths” is itself a truth claim, it inherently contradicts itself. Practically, it is a mask for cowardice and disengagement from life, in almost all cases. If nothing is true, then there is never a need for sacrifice.
And actually, that leads to an interesting idea: if we do not sacrifice ourselves, we are forced to sacrifice others. If I am not working on myself, I am bound to be taking from others.
It is obvious what the Cultural Sadeists take, but what do the Sybaritic Leftists take? They take meaning. They take the higher purposes of life beyond simple comfort and pleasure. They see safety as an end in itself, and danger and risk as inherently undesirable, because they know that is where the learning is, and they do not want to be reminded that life is always a fatal experiment for all of us.