I'm not a doctor, but back when my doctor and thought I might have true bipolar disorder (vs. as it turns out "depression of a bipolar nature" that only goes manic when I take the wrong drugs), I studied up on the disorder.

What's prescribed for people with is are "mood stabilizers" (and all of them but the "gold standard" of lithium were originally developed for other indications like psychosis and convulsions). So people who have true bipolar disorder by definition have manic phases, and they need their mood stabilizer all the time to deal with both that and depression phases.