take my brain dump

most people think that the red soul is DETERMINATION.
DETERMINATION, according to Alphys, is the will to live. Humans have more of it then monsters, which is why human SOULS can persist after death. human bodies are made of physical material, while monster bodies are made of magic. so humans need to use the bathroom and decompose and yadda yadda, monsters just aborb energy and turn into dust.

so there are 7 soul types.
Bravery, Justice, Integrity, Kindness, Perseverance, Patience.
And the RED soul.
The red soul is sort of the default trait. it's what a human is born with before they gain a personality. frisk is like nine years old in UT, so it makes sense why a nine year old would have so pure DETERMINATION following this theory. i also think the idea of every human fitting in one of 6 heart-shaped cookie cutters is stupid, and that every human is a mix of colors. this is also why it took so long for each SOUL needed to break out of the underground; you need seven PURE human SOULs, and only children, who haven't figured out who they are yet, possess.

I think the reason why humans don't dust when they die is because blood is liquid DETERMINATION. it's said that flowey's flower/body/ykwhatimean was prepared by injecting DETERMINATION into it, and it would make sense if it's literally blood.
susie and sans bleed because they're both descended from humans. sans most likely not directly, but susie most likely has a human parent and that's what made her an outcast. she lived with this human parent in human communities, making her seem like a (literal) monster to them. monsters are also just closer to humans in DR, so it makes more sense for them to not be able to use magic at all or as easily. it's also possible that they just have forgotten how, since we've seen no mention of SPARNG or any other battle elements outside of the dark world.
"how come gerson is dust, though?" i hear you asking. gerson is most likely pure(ish) monster, but in the 10th anniversary stream, gerson mentions a 'beautiful human'... Alvin's mother in DR.
though it's also possible that susie and sans are unique, because monsters like noelle(even though shes probably pure monster too) don't know what blood is. and when i say blood, i mean like, real blood. you can't realistically pretend ketchup is blood, and she mentions a prank Kris did on her where they did exactly that. so while she knows what blood is, she doesn't bleed herself.