The way that we understand the world around us, and situate ourselves and others within that world, depends upon how we've been socialized, a process that occurs when we're young. We learn about the different types of social categories, and which represent "us" and "not-us." Some identities are considered socially unacceptable, or deviant. Mental illness is one such type of identity, and it's seen as taking over the entire person.
To learn more, check out this blog post on Mental Health @ Home:
https://mentalhealthathome.org/2021/07/12/prejudice-implicit-explicit-beliefs/