Why do schizophrenics only imagine bad things?

Schizophrenia is a mental disorder characterized by abnormalities in the perception or expression of reality. It makes it difficult to tell the difference between real and unreal experiences, to think logically, to have normal emotional responses, and to behave normally in social situations.

My question is, why do schizophrenics (not on medication) only seem to imagine things that would be considered more negative than positive?

For example, if they are paranoid (constantly afraid of bad things like the government implanting tracking devices/thought reading chips in their head or someone following them on the street wanting to kill them), then why can’t they constantly imagine/constantly be happy about something good? For instance, imagining everybody around them loves them because they are a famous celebrity. Why don’t they go around offering their autograph because they think everyone is an adoring fan? (Just one example among many.)

I understand that it has something to do with chemical imbalances in the brain, but even most people who are bipolar swing from the depths of despair to rapturous highs.
So why can’t a schizophrenic ever have a different perception of reality that happens to be more positive than negative?

And I’m not saying that they can’t or that it’s impossible. I’m just saying that it seems to be pretty rare.