Because that justifies why you would specifically need to claim Asgore to escape.If Boss Monster souls couldn't persist, this would have been impossible. In one of the neutral routes, Asgore tells you to take his soul and cross the barrier, until Flowey destroys his soul anyway. What was the point of even telling the audience about Boss Monster souls being able to persist for a short time after death, anyway?.Without the reinforced attachment, Asgore would lose them too quicky. There is some Fridge Brilliance, because the bodies of the dead children are conserved in a little crypt. We used to burn in ceremonies the bodies of the deceased in the past, so they can move on quickly. Human souls have the problem that with death of the body, that all the living tissue needs to dissolve first into non-living matter for the attachment to go away, and when conserved it will take more than centuries for this process.

The body of a monster dissolves pretty quickly in non-living matter, allowing the soul in question to rise quickly in higher frequencies as there is no anchor to hold them down.

The containers for souls also have to do with that, as they are a cause of attachment.

It could be just the case that these souls just are capable of changing to higher vibrant frequencies, thus vanishing quickly in higher dimensions (thus becoming out of reach, physically as well as visually).

Injecting the flower that contained Asriel's essence with Determination revived Asriel's consciousness within the flower, but not his soul, which died shortly after his body. When Asriel died, his dust dispersed over the flowers in the garden, which were later used by Alphys in her Determination experiments. It's mentioned in a library book that monster funerals involve spreading their dust on something they loved in life, which then allows their essence to live on in that object forever. So it's entirely possible that Asriel's 'essence' is simply his memories or an echo of his mind or something like that, not his literal soul. You see this when you kill Toriel, her soul persists for only a few seconds before 'dying' itself.

Boss Monster souls also only persist for a short while.

It's possible that unholy void of this being's soul destroyed Asriel's, leaving it in shambles when he died without bringing slaughter to the human world.

The thing is, Asriel tainted his soul with Fallen Child's.

Why? Is his SOUL not the very reason he reincarnated? Asgore and Toriel) can persist after death, and since Asriel is the son of two Boss Monsters, we can assume he inherited this ability, which is, theoretically, how he'd be able to reincarnate as Flowey during Alphys's experiments, because his essence would still persist in the flowers of Asgore's garden however, when he does reincarnate as Flowey, he doesn't have a SOUL.

It's said that only the soul of Boss Monsters (i.e.

Undertale the six souls