Firstly, because saying you are a thing doesnāt make you that thing if you arenāt. If you say youāre a vegan but you regularly eat meat, then youāre not a vegan. If you claim to be a Democrat but vote for Republicans, youāre not a Democrat. If you say youāre a doctor but you donāt have a degree, you are lying. Definitions are descriptive, not prescriptive. You are either male or female. As such, you are free to dress and express yourself however you want, gender-conforming or non-conforming, but that doesnāt mean you belong to an entirely different sex category.
Secondly, if youāre using ānonbinaryā to describe gender (i.e. gender roles), not sex, then itās a worthless term because no one is binary. I challenge you to find even one person who is either 100% masculine manly man stereotype with absolutely zero āfeminineā traits, or a woman who is 100% āfeminineā. If you can, then, congrats, literally everyone else in the world who falls between those two extremes is nonbinary. But you canāt, because what is āmasculineā and āfeminineā varies from culture to culture so you donāt even have a solid benchmark with which to work. Which makes everyone ānonbinaryā. And if everyoneās ānonbinaryā, then the word is useless.
Being pro gender-abolition and being nonbinary are incompatible because expressing that you are nonbinary enforces gender roles. āI do not subscribe to female gender roles, therefore I am not a woman!ā is far less progressive than, āI am a woman, and I will behave and dress how I like, even if it violates sex stereotypes.ā
And lastly, itās not that we ādonāt believe inā nonbinary people, like theyāre Bigfoot or something. They exist. They just arenāt what they claim to be. Theyāre each either male or female and pretending to be something else.