Why is shirk the one sin that can't be forgiven?

Assalamu alaikum, everyone! I'm a Muslim sister, and I came across a note I wrote about shirk that got me thinking. If anyone could help me understand and clear up my thoughts, I'd be really grateful. Just trying to deepen my knowledge :) "Why is something that doesn't cause any physical harm in this world considered so serious? From a spiritual angle, you could say that disbelief might lead to immorality, but that doesn't always make sense. You don't need religion to be a good person, and even people of different faiths share basic morals. Humanism too. So how can awful things like rape, genocide, or corruption be forgivable, but shirk isn't? Is it because faith is more about the state of the heart and soul, not just actions? Still, if someone lives a righteous life but associates partners with Allah, they're destined for eternal hellfire. How is that merciful or kind?" Sorry for rambling, but this really has me puzzled! Jazakum Allahu khayran in advance.