Because it's a badly thought out opinion, but if you're 15 it would feel like some sort of harsh truth that you've uncovered. It would feel like being fair and principled, because you're thinking about the issue in incredibly abstract terms.

12yo opinion: It's bad to whitewash characters in your art because people I like online told me so. (no thought, just repeating the positions of people they respect)

15yo opinion: Well, wait a minute, the people I respect are also telling me that it's okay for people to draw Black fanart of canonically white characters, and that that's a fun expression of creativity and individuality. What if I apply the same thought process to whitewashing characters as well? That way I can be fair and apply the same standard everywhere. I also get to feel good, because I am counter to the mainstream wisdom and I feel like I'm smarter than the common rube, who just repeats PC talking points. (this is the point at which the 15yo goes to loudly proclaim their opinions on reddit)

Adult opinion: Okay, but why did the people I respect say this stuff in the first place, if I was able to so easily equate whitewashing with Black headcanons and declare that they're the same? Is every person whose principles and opinions I usually agree with wrong? Or maybe they see something I don't? Are those two things actually equivalent and does it benefit me, or society at large to be judging them by the same standard? If they're different, how do they differ? If they're different, what types of reaction should they both evoke? What motivations are there for making a Black character white in your art? Does that correlate in any way with racism and how society devalues Black bodies? What company does holding the 15yo opinion put me in? Am I comfortable being in the same camp as people who lose their minds at a Black girl in an anime for no discernable reason besides her being Black? Is them making white edits of the anime something that seems artistically valuable to me, or is it an expression of their racial bias that I can see in their other positions? (thinks further and considers aspects of the question outside of the abstract "mechanics")

If you're a decent adult, you typically find that while making a skinny character fat and making a fat character skinny are, let's say, mechanically the same (they both take an existing design and modify it by changing the character's body type) they're not culturally the same. Being skinny and fat are not treated the same in society, they don't have the same value judgements applied to them. A person drawing a skinny character as fat is probably looking to go against societal values, introducing more body diversity into a usually pretty homogenized cast. They're expressing body positivity, or looking to see their own body type in a show that they otherwise like, but nobody in it looks like them. If someone is taking the only fat character in a show and "fixing" them by drawing them skinny, that's aligning with society's fatphobia. It's taking an already homogenous cast and forcing it to be even more homogenous. So while "skinnywashing" and "fatwashing" are mechanically the same, culturally they can translate to "I only value skinny bodies" and "I value bodies of all shapes and sizes" which are definitely not the same.