Why do you reincarnate? Let’s take the Buddhist view. The cycle of birth and death (rebirth) ends when you let go of craving for existence, craving for sensual pleasures, and ignorance (not seeing things as they truly are). (aka the Asavas.) So, that means these things are the conditions for rebirth. And they are the source of karma in your current life. Do you dream of being rich, healthy, strong, pretty, etc? That’s the first one. Do you have fantasies of sex, dream of the next meal you’re going to make, or just want to see the sunset from a mountain top? That’s the second one. Or are you convinced your individual self is separate from everything else and/or has some permanent existence? (a rough inaccurate summary of ignorance)

If any of these things are true they are probably focused on your current life. You probably don’t have fantasies of sex with odd slugs in proxima centauri. You probably don’t dream of being a brain parasite. The conditions for the next rebirth are set up by your cravings. And, most likely, you crave something distinctly human. Ergo, the next birth generated by your karma will most likely be human.