Meaning is something that is imposed by an observer. For something to have meaning, there has to be someone it has meaning to.

Religions typically offer a supernatural observer (usually but not necessarily a creator god) to play that part.

Atheists don't have the luxury of simply defining such an observer into existence, so they fall back to that which already exists or has a reasonable chance of existing if things go as planned: themselves, friends and family, their offspring, other humans, whatever comes after humans, maybe even some other species that is luckier than us and stumbles upon our remains.

But because meaning is subjective, it is also contextual: ultimately everything may be meaningless because there is no absolute observer ("ultimately" kind of implies there is no observer left anyway), but in the meantime there are potentially billions of people whose lives you can affect -- and many billions upon billions more if we as a species survive that long.

Talking about religions as if they were all the same or at least similar enough to lump them on a single pile is dishonest. That's often how apologists try to define their deity of choice into existence (drawing semantic arguments that define "god" in such a way it becomes a logical necessity) but there's no path from "there is something I shall call 'God'" to "everything in the Bible/Quran/Talmud/Necronomicon is true" short of circular reasoning ("because the Bible/Quran/Talmud/Necronomicon says so"), special pleading ("of course all the other religions are just superstitions") or "divine revelation" (i.e. non-repeatable personal anecdotes).

Speaking of "the meaning of (human) life to a rock" would be patently ridiculous: rocks don't have a capacity to impose meaning on anything humans do and even if they did we wouldn't be able to tell nor have any incentive to care.

Speaking of "the meaning of (human) life to Bob" likewise is only worth anyone's attention if we have any reason to actually care what Bob thinks.

Even foregoing the impossibility of proving the existence of a god or gods (given how awkward they are to define consistently across religions and how being able to actually demonstrate their existence would by definition refute their supernaturality) why should anyone care what they think about life, human or non?

Many religions like Christianity solve this problem by making their god vengeful. Don't do what Yahweh wants? Experience unpleasant things for eternity (whatever "experience" or "unpleasant" means in eternity). Do something he likes? Experience eternal bliss (whatever "bliss" means if it is eternal). Various sects disagree on the exact nature of the punishment/blessings and what is necessary to deserve them.

Either way, you have to wonder what difference it ultimately makes: if we're talking about some kind of post-physical eternal state of "being" without bad things, what's the value of good things? If there is none, what difference does it make whether you're blessed, punished or simply cease to be? It almost seems like an elaborate con based on all living things' natural fear of death and non-answers ("What was before the universe?" -"Turtles." "What was before turtles?" -"Turtles were always there. It's turtles all the way back.").

Also, with regard to the Christian god in particular: going by the Bible, Yahweh is not exactly the kind of character whose opinion I would value. The Old Testament is full of him commanding his followers to murder, pillage, rape and enslave others, not to mention the bloodshed and terror he allegedly inflicted directly. Even the New Testament paints a less than rosy picture of him. If Yahweh is real, he would be worse than all human tyrants of history combined and a perfect example of how absolute power corrupts absolutely.