Let me at least try to lift the value of humanity from being an assumption to being a rationale.
Life seems to be pretty rare[1]. At least on the order of once per solar system. Nervous systems capable of thought also seem rare, based on terran history. Analytical thought a fortiori. We are very possibly the only species capable of understanding the universe within at least a rather large radius. Inter-planetary life is really freaking difficult, but it is a complete cakewalk in comparison to interstellar living, let alone intergalactic life. How many shots does this universe get at understanding itself before the whole thing heat-dies? And, who knows, maybe even doing something about that heat death.
I think that matters. Life matters. Consciousness matters. Beyond it just being ours[2].
Earth is a paradise, but a fragile one. If earthly life (not just humans, any life) is going to end up spreading across the solar system, we are by far their best shot.
Perhaps if all life were destroyed nothing would be lost, because things end. Perhaps the entropy of a dead child is no more significant than the entropy of a cooling car engine. Perhaps the existence of the universe is itself irrelevant. After all, what (and to whom) would it matter if it ended? Starting with different assumptions gives different conclusions. I don't think there is any way to prove either axioms correct. But whether you agree or not, the assumption is somewhat different to the one you identified. And there is – at least – a 'why'.
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[1] The question of the prevalence of different complexities of life is – in my experience – a perfectly tuned discussion for 'well actually's. All of my second paragraph can be quibbled. But I don't think any step would invalidate what I'm doing with it.
[2] In fact, I think is likely that the intelligent life we end up sending interstellar will be descended at least as much from human technology as human biology, perhaps overwhelmingly more so. But again that is not directly relevant to my point here.