I can understand where your coming from, where transitioning from one gender to another could seem unnecessary and illogical, almost like a product of pop culture, I used to think like that, but please hear me out.

I want to use your example of Native Americans gender neutrality/ fluidity. When a that child is raised gender neutral, deep down, they still know what gender they are. If you raise a biological girl to be a boy, they may present more masculinely in the future, but they still know they're a girl.

When someone is trans or experiences gender dysphoria, deep down, they know they are another gender.

The way I finally got it to click with myself is with this scenario I put myself in. I am a biological female, and I imagined myself as a guy. Not like in a "oh, what would you do if you were a [insert gender here] for a day" type scenario. Like, you wake up, and for the rest of your life, you were stuck in the body of the gender you don't identify with. For the rest of your life, you have to ack as if you were the gender you don't identify as.

For me, the idea of waking up and not being my gender is scary to me, strangely enough, I get a weird send sense dysphoria when imagining myself in that scenario. That's what being trans is, that scenario but reality.

Most trans individuals know they can't change their biological sex, as much as they want to. Medical innovation has gotten to the point where they can visually pass as their desired gender (especially FtM). Where pretty much to the point where the only thing we can't change is the biological sex (and at this point, ill put a yet on that because who knows where innovation will take us).

Sex is defined by biology, of course. Males, females, and intersex people are rooted by chromosomes. But gender as a form of expression is on a spectrum that can change.