Because that is just not how anything works. That is against universal laws of nature. A man is a man, and a cat is a cat. A man will never be a cat, and a cat will never be a man.
Do you think/feel when a soul becomes immortalised - that that being can then appear as anything they will to be in physical reality? So, when a human being becomes an immortal, then they can appear as anything in this world?
People who can see energy - describe the soul as a connection to the Devine. It is more like a conduit of encoded light information to source that then flows through the Taiji Pole , or Shushuma, rather than a one off blueprint that is set in stone, for a one off type of reincarnation. And those that have transcended matter are able to, in a figurative sense, move or remold this divine connection, so that different parts of the whole of the divine are expressed, or channelled. That would be Shift Shaping.
For those that aren’t immortal, the soul connection to the divine, is moulded prior to reincarnation. In Daoism they say that the eternal soul is moulded by the WILL or INTENT of heaven, - Zhi Yi Tian
In Daoism
You have the DAO
Then the Shen Ling
Shen Xian (which is the eternal soul that reincarnates - and is connected to the Devine)
Zhi Yi Tian (the will of intent of heaven shapes the eternal soul into a human soul)
Yuan Shen (where experiences are recorded) it is the yang part of the soul
Shen Zhi (the acquired personality) the yin part is of the soul
So you see energy steps down in vibration or dimension, from the DAO to the earth dimension. So the energetic nature of the eternal soul is transformed in to a human soul.
The human soul resonates with the vibrations of matter, but the eternal soul is beyond matter, and it is this which can be moulded or developed into different types of souls, by passing through the ZHI YI Tian, which is the will and intent of Heaven
It is a less ‘physical’ way of seeing things. The irony, as always, is that beliefs, obscure our energetic perception, and therefore fix our awareness, this is as you said, this type of thinking, in fact any thinking (because this is a projection of mind, it is not seeing energy directly) fixes the soul into one position of awareness, or mould of perception, which fixes or limits the expansion or connection of your soul to the will of the divine.
I’m not saying I subscribe to the above, I wouldn’t rely on any beliefs, just go on direct experience of your own seeing, and be open to seeing energy directly.