Because it’s not a “normal” understanding of past life beliefs, therefore it’s too weird and wrong. People react to things they don’t understand with fear or anger a lot. Alternatively, they get a little overeager in their protectiveness by thinking one or two particular religions are the complete authority on past lives of any kind because they happen to share a basic concept.
The generally accepted ideas about reincarnation are just weird to me. We can see the great number of ways in which nature operates in cycles, whether its the life cycle, the water cycle, the solar and lunar cycle as well as the seasons.
We know that energy can become matter and vice versa, we know that neither matter nor energy is created or destroyed, and we generally consider these to be physical laws of nature.
And yet we believe that our spirits and/or souls are somehow above these cycles and rules. Most religions on Earth believe in single life existence, and the ones that believe in reincarnation seem to append strict sets of rules to it. Some say humans only reincarnate as humans, others say you can only reincarnate into a greater life form.. Which begs questions, like “why”, and “what defines a higher lifeform”? Most of the rules (for most religions actually) are anthrocentric.
I personally believe you can reincarnate into anything that has ever existed or will exist at any time it may come to exist or has come to exist. You could even be the first of a kind of being being incarnated for the first time, thus creating the thing you are being incarnated as. For example, there has to be the first dogs, cats, foxes, dragons, etc. And there is no specific “forward movement” into “higher beings” because the mulitverse isn’t taking measurements of what is is lesser and greater.
Most of the ideas about reincarnation and even the afterlife in general, are highly anthrocentric, and in some cases, culture-centric. What does this mean? It means these ideas have been formed under the presumption that humanity is the baseline and center of existence, that humans have some kind of superiority over all other animals on Earth, to even the multiverse. I do not believe this is true. And I do not think the multiverse does either.
Logical and observational evidence says all the matter that makes up ‘us’ has been part of many other things over the course of the life of the multiverse, so why does humanity believe their spirit/soul is somehow different? While I believe a spirit has a certain ‘nature’ that draws it towards being oriented with a certain thing (such as a human, a fox, a pony, etc..) possibly resulting from frequent reincarnation as that thing through familiarity, that there is actually nothing stopping that spirit from becoming something else in another life, with everything meaningful that comes with it.