Well, a huge number of people in the US alone live with mental illness, or they live with someone who has a mental illness. It’s not like they’re Inhumans or robots from the future, they are real people, these are real conditions.
If we write a character with a certain ethnic background, or we write about a certain location, we are expected to do some research to make it truthful and credible. Why would we not want to do that with characters who are mentally ill?
And I feel like the question is loaded. Most people aren’t asking for ‘sympathetic’ portrayals exclusively, they are asking for truthful portrayals to balance out seventy years of mean and untrue stereotypes. Which seems pretty fair to me.