It’s like … mythology or something, and the planet(oid) being anthropomorphized.

I mean, the planet(oid)s are named after gods.

The personification of its classification is probably related to the exclusivity of the title and “bigger is better” mentality. Since every life form has a taxonomy for domain to species, there’s not really an exclusivity to each echelon. I don’t imagine anybody really thinks like this meme below, for example:

Do people also view kingdom, phylum, class, order, family, genus, species as "ranks" of some sort, with some intrinsically greater value being given to some over others?

Well, for humans we most certainly do

I think it'll matter a bit more once (if) we get to explore our solar system for real. I feel like right now the concept of "planet" is still rather distant in our minds and a lot of people just base it on vibes

Hard not to anthropomorphize things when they’re named after our gods

Thing is everyone has one of those.

Compare it to non-sentience, sentience, and sapience, to properly anthropomorphize it.

Planet Y

In case it's a dwarf