Why is it that people are so stuck on there being just 'female' and 'male'?

I find that some people can be quite judgmental and see things in black and white. There can only be female or male. They don't see what we see and how there can be transgender, genderfluid etc. There is nothing wrong with the gender you have and I would suggest trying to ignore people that try and push this belief on you and educate people with this belief.

Some people are just raised to think that what we are born as, is what we are. There is no in between. It all depends on how people learned about sexual orientation.

We as a human species love categories. We use categories to quickly understand what we are seeing and experiencing so that we can make snap decisions based on them. Most of traditional western society has dictated that there is a "boy" category and a "girl" category. This is what we were taught, and this is how we understand the world. We live in an "either-or" society, and fluidity (in gender, sexuality, ideals, etc.) does not fit into it. When a new idea is pushed into our engrained values, it is difficult to understand or accept. This does not make those who don't understand inherently "bad" people, it just means they were taught to view the world differently.

It's a matter of cultural stereotypes. Society has created male and female as two fixed category in which everyone must identify, and many people internalize this mentality. But you're free to live your identity openly, being non-binary and non-conforming is a totally valid identity, and you should be free to live it openly. Never lose hope, accepting and open-minded people are there, and there are some who are willing to listen and learn about realities they didn't know!

It's actually very hard work to get and stay within the boundaries of what is considered acceptably "male" or "female," and a lot of people secretly feel like all their effort would have been wasted if we toss out the rigid mandatory binary. Also they're not used to thinking about gender and it confuses them and makes them feel uncomfortable.

As we are taught from birth that there is only the binary system of Female and Male and how they are biologically defined, people are unwilling to let that idea change and to accept new things. It can be because of belief, fear, or any other reason that holds them back from acceptance, people reject change and reject things different from what they know.

Because they are ignorant. Some people never understand. or quite simply don't want to understand. they are fearful of difference

People tend to like to categorize. One of the easiest things to categorize about a person in most peoples' minds is gender. As time moves on and the more people let it be known there is a spectrum of gender, this may change. Until then, people find comfort in being able to categorize things like gender and race, and find discomfort in ideas they don't understand.

Probably, because for most of history that was all there was. Gender identity issues are quite recent when cast against the long, long, long history of two unchangeable genders.

I don't think it's that they are "stuck", I think maybe they haven't been exposed to the education of all possible genders. What if instead we didn't ask pregnant mom's what they were having, a boy or a girl? What if we waited to see how that baby identified? People like boxes, they like neat. Unfortunately sexuality isn't as neat as this or that. There's lots of stuff in the middle and way past the ends of boy/girl.