If human souls can simply become monster souls like that, then it means they're more or less interchangeable here.

[...Right, right, he might want to explain- ]

Human souls are far stronger than monster souls, where I'm from. If that isn't the case here, and monster souls are on par with humans', then they can live through about the same amount of...well, everything.

So we can't die for the same reason monsters can blow themselves up and just faint - our souls can last long enough for our bodies to reconstruct themselves. If someone like...oh, I don't know, Papyrus or myself tried that back home, we would probably melt, and that would be disgusting and not photogenic at all.

...Basically, it explains why we don't die when we're killed. I think it's nice.