Basically the reason cissexism even exists is because people think gender and sex are the same thing. Since they think gender and sex are the same thing, there is no line between “man” and “woman” and  “male” and “female” because people don’t see the difference between gendering and sexing.
Like if people realized they’re not the same, cissexism probably wouldn’t even exist (along with the gender binary)

I also think that society socializes us to assume certain attitudes on how to treat “men” or “women” and if you don’t behave like you are expected to then your actions are often misread. You are also often not treated as you would want to be treated if you’re not cisgendered.