I think its because most NBs (at least from what I've seen) are extremely narcissistic. With TIMs and TIFs you can at least understand that they want to be the opposite sex. Non binaries claim that they're neither men or women but this special gender that (despite having no basis) only they know about. Unlike TIMs and TIFs who mostly try to "pass" as the opposite sex, non binary requires no "gender affirming surgeries" (unless its a woman who has her nipples removed) and no gendered clothing to pass. You can literally just claim the label and people will have to bow down to you or else get sued for a hate crime. Honestly, non male or female gender identities have always baffled me as to why anyone on the left takes it seriously since nobody on the TRAs side has ever explain why we should believe someone who claims the label, but does nothing in terms of transitioning. Especially for something that was literally made on tumblr and has no basis neurologically speaking.