Because being the agender I generally feel inside is effort I'd rather spend elsewhere I guess. Gender is performative. I'd rather not perform at all, but there's no such thing as not performing, an unperformance. So I passively perform the identity put upon me, feeling powerless to step outside of my experience nor how society experiences me.
Heteronormativity doesn't make sense to me. To my subrational mind, everyone possesses an ungendered essence, which is filtered through and expressed by social and physical constructs like gender and whatnot. Merely signs or signifiers for the essence that is the person with whom a connection might be made.
Rationally I don't think these conceptions are particularly true, but they kind of describe how I subconsciously understand the world.