I think the most common explanation is that people are reincarnated as animals sometimes and animals into humans
But here's the thing.
How does a spider know how to spin a web?
How does a woodpecker know it can use its beak to drill holes in wood?
How does a white rabbit whom cannot even see what it looks like knows that it is the same color as snow?
It's all "instinct" right?
So if reincarnation is real the only reason a spider knows how to spin a web or how any animal knows how to do anything in general that means a spider dies and becomes a new spider and it remembers what it use to do as before.
So if a human reincarnated as a spider, how the forget is a human going to know what to do as a spider? It had no previous memory of being one.