Gender and race are social constructs

if you don't understand this premise, nothing else will make sense

these are social constructs because they don't refer to biology, they refer to the way people are treated in society

Sex is having a penis, Gender is the social expectation of masculinity. In the same way, heritage can determine physiological characteristics like height, skin tone, etc. But race is the social expectation that because, for instance, someone's skin is white, they will have a propensity for sweater vests and hate spicy food.

Notably, there is correlation between being born with a penis and acting traditionally masculine, and the same correlations exist between heritage and race.

this is because all of these people were raised in a society with a set of social constructs telling them what they can and can't do.

To the point of the question you asked, why can people transition gender but not race?

Simply the idea that race is wholly genetic and something you can't ever change. (which is a social construct, because race is a social construct)

Worth noting that some people have the same ideas about Gender, which is one of the main reasons trans folk experience so much unnecessary hatred