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

Great question. I think the answer can be found by understanding that humans naturally like to build "boxes" or "labels" in order to better understand the world. This can be both helpful and harmful. A great example of this being harmful is the strict categorization of genders. Gender norms are rooted in many cultures, often giving strict roles to "males" and "females." However, the reality is that gender is a social construct, meaning that they are simply labels created by humans to categorize people. It's hard to change social constructs because they are often rooted in society for a very long time. Gender constructs have been a part of human society for so long that it is ingrained in many people's minds. This is why it has been so hard to introduce the idea of gender-fluidity or the idea of being non-binary.

Because this is the way they've been brought up. When something is brought up in someone's mind and it stays there for a while, it's harder to get rid of. Some people, and I think we're all guilty of this to an extent, like to confirm the reality we're in instead of trying to see someone else's point of view or reality. Some people are stuck on there only being female and male because that's all they ever know. Having another gender would invalidate that and science/biology is used to reinforce this. It's more of how logical, feelings and reality compare when stating things like female and male because they may see genitalia and not how someone identifies, which is on a spectrum. Lastly, not differing between sex and gender.