This makes NO sense.
"How come only cis people get to BE their gender?"
Because they were born that way.
Literally that's it. It's the same reason tall people get to BE tall. You can call yourself tall at 5" if the rest of your family is 4" but to the rest of the world you're short. You can lie on your passport, you can even tell everyone online you're super tall and hey, if you never see them in real life they may even believe you.
But the second anyone actually meets you, the lies you tell yourself and those that issue your passport really don't matter.
Because your genetics make you
male short and anyone who says otherwise is humouring you or just trying to placate you so you stop making a scene in the Gamestop.
And really, now that I'm drinking beer and feeling a little ranty, I think that's what bothers me the most. I've heard more than once "Well what if people misgendered you?" And the truth is I don't care and never will because I'm not insecure about who I am. I know who I am and someone calling me "Xir" or some shit won't change that. And I get that in that sense I'm lucky, there are a lot of people out there quite sensitive to name calling but with Labelle and the likes it's actually never about malicious name calling; it's about guilting people into validating them (some going to extreme measures like being dick out in the women's change room for it) in hopes that repeating it enough will make it true, or a tleast finally convince
themselves enough of the lie for it to feel real and rid them of their gender dysphoria.
The problem is when others stop or aren't around to say it, the reality sets in and that's where the rage, depression or even suicidal thoughts really set in (depending on the person) because no matter what you do, no matter how much you scream at them or guilt them or insult them, other people cannot fix the issues you have with yourself.
But like with the morbidly obese and losing weight; fixing that shit is hard and takes A LOT of work and inner strength, and also like with the mordibly obese most of them are looking for that quick outside fix (surgery, anyone?) that will do all the hard work for them, which is why it usually fails and the morbidly obese regain the weight, and the post op trans kill themselves.
And don't get me wrong! When I say it takes a lot of work I don't mean for them to even accept themselves as their assigned gender, although that would probably be the healthiest route, I mean to even accept themselves as trans women/men. To be able to look in the mirror and go "Yes! I am a woman (if they're a transwoman) no matter what others say!"
Which means also coming to terms with the fact that people WILL misgender you because even post surgery you're probably going to clock as trans, accepting that embracing your true self means your dating pool is limited because of how you present yourself, your fertility and the state of your genitals, and understanding that you will have to accept certain boundaries set by your cis counterparts.
Is it fair? Probably not in everyone's minds, but we're a species built on community and harmony in the community means there will be some give and take.
Think of it like moving to France when you only speak English. It's not your fault you were born in an English speaking household, or that your schooling didn't offer French classes, so you'll have to accept that you're going to have a harder time living there. Most people will understand a bit of English, you'll be able to get by, but you'll be expected to learn french even though your accent will always let people know you are Anglophone. You don't scream like a child throwing a tantrum that everyone start speaking and writing in english to accommodate you, if France is where you really believe you belong then you put in the work to integrate.
TLDR; No one will do the work for you; fix yourself. You cry like a spoiler, overprivileged child throwing a tantrum and then wonder why no one responds positively to it. No one is asking your to change your identity, just asking that you act like an adult.