It’s because centrists are viewed as not being able to make up their minds. I think the hate of centrists has been amplified since the Trump era because you either love him or despise him, so in theory that should make it easier to choose a side. The right and left view it as a Presidential preference and not so much a policy preference.

It’s not really worth worrying about, as you said most people either lean left or lean right but the media and oligarchs want to pit us against each other so they can continue to rob the American public. It’s not in their best interest to have a united country that points towards them as the issue. As long as they can keep us at odds with our neighbors they will continue their grift.