I think people get so mad over it because of wanting to
A. preserve their own culture (not in the case of white people though, they seem to despise themselves) or B. preserve your original culture
The concept of finding another race more appealing than the one you are deals a lot with historical biases, like how asian people tended to value paler skin back in the day and are finding it difficult to break the cycle, or about colonization and the like. It also deals with "cultural appropriation" in which someone adopts the mannerisms of a culture not their own without respecting the spirit of it... or something.
Also, being transgender and transrace are two completely different jars of dirt. On one hand, a transgender person can just get plastic surgery to pass, wear different clothes, that's it. The only difference between a trans person and a pre-trans person is the way they are perceived by society. Passing is good enough to be accepted, take Blair White for example. I don't know of anyone who'd still refer to her as a he despite her being open and honest about her biological sex.
On the other hand, being trans race would require extensive work on skin whitening (which is dangerous) or "colourface" (which is offensive) and cultural appropriation (also offensive). The difference between someone who was naturally born into that culture and someone who transitions into it would be immediately noticeable in the lack of nuance and one could easily find themselves being outcasted by their chosen society. A weeb will never know the japanese culture like a native, and just "passing" isn't good enough.
I understand that, but if someone migrates from another country to America, I consider them to be American with legal citizenship. Even if they aren't familiar with my culture. If someone wants to be black, then I think they just need to be serious about it and learn everything they need to know about our culture and our struggles. Just like being in an interracial marriage for example. I don't see it as cultural appropriation if they are serious about the culture and don't just wear it as a costume.
For trans people, I see someone as the gender they identify as regardless of "passing". Trans women just need to be aware of the same stranger danger that bio women go through and probably aware of even more in terms of jerks targeting trans women.
I noticed a lot of people saying it's self hate or racist, but I don't think it's racist to want what someone else has along as you are serious about learning that culture. Then I think it's fine if someone white wants to be black.
If a black person wants to be white, People would say it's because of slavery and the horrible way black people were treated back then, how white was considered 'beautiful' and black 'ugly'. Something like that.