> Or because there's significant incentives to split groups in order to name more species/families/orders/etc?
You can ask yourself why such an incentive wouldn't exist for finding new human species etc. The interesting part here is that humans are treated differently, not whether one is better than the other.
According to the article, before the 1940s, it did. Judging by the list of species in the image with spell-check underlines, anyway. Something weird happened in the 1940s.
To expand on this, a lot of intellectual energy was spent on expanding and properly defining what was meant by "Human" in order to preclude any perverse interpretations, such as what the nazis did, and to shut down a lot of eugenics nonsense.
Reading natural sciences and eugenics books written prior to the 40s is fascinating - there's all sorts of casual racism and talk of different cultures of people as if they were completely different species. Global communications improved after the 40s, and it became progressively more difficult to pretend to be a scholar or intellectual in public if you had racist or supremacist biases. The current state of liberal acceptance and human rights in the modern world is part of a trend that was an incredibly radical departure from almost all human culture prior to wwii.
You can ask yourself why such an incentive wouldn't exist for finding new human species etc. The interesting part here is that humans are treated differently, not whether one is better than the other.