"why do so many non-christian people seem to completely reject the mere possibility of Jesus, or even just the holy ghost, or some kind of holy grace connecting lifeforms?"

Because it's in the name - after-life, souls, spiritual energy - those are religious concepts. If you're not religious you don't believe in those things. I guess people might become atheist from different perspectives, but if the perspective is scientific/materialistic, then the point of it is that you don't believe in extraordinary things without any evidence, especially if you wish them to be true. Is the afterlife possible? Sure, I guess. But I also think it is impossibly unlikely. Does that make me agnostic? I make the assumption there is no god and call myself an atheist.

It is also possible we are all actually alien simulations. Does it make sense to call yourself aliensimulation-agnostic and to live your life believing both human and aliensimulation versions of reality are equally possible?

I think you're falsely equating the statement 'I am 100% sure that there is no god' with someone labeling themselves as atheist.