Honestly I think it's because it feels like society prefers them for the most part. You see all of the time, especially on this site, men or women saying they "won't date a black person". It may just be their preference and that's fine, you can't help who you're attracted to but when it comes across as the most preferred then some white people are naturally going to feel superior.
Personally I think Asians are the most prefered by everyone, it's difficult to even find an unattractive Asian person lol
I'm as white as you can get and I don't feel superior to anyone in terms of looks and I actually think Asian women are some of the most adorable women on the planet. And that black women have absolutely beautiful features and the best shapes. But of course I think white/latino women are beautiful as well.
Not all of us are in some race appearance competition with each other.
We don't - that's why the appropriation of the features of Black women is so widespread. However, over the years, white-dominated culture has promoted the standard of the European woman as the pinnacle of beauty; straight hair, pale skin, a thin figure, blue eyes, and a small nose are considered more "beautiful" in much of the world. Overall, Eurocentric beauty standards have been pushed for centuries, but this is starting to change as women of color become more prominent in the media. This leads to "trends" such as full lips and thick eyebrows, appropriated from Black and Latina women, respectively. I don't mean that no white girls have full lips or thick eyebrows naturally, but it tends to be the case that these features are less common with us.
Personally I don't feel that way. Maybe you should ask why you feel like white people think they are superior in terms of their appearance. I am white I tend to prefer races with olive tones, then whites and then blacks after that, but that's just me.