The poor capitalists with all the money and power, making the decisions at the highest levels, guiding society's direction... can you imagine they might be responsible for the shittier and shittier turns our world is taking?
Class is the shape of power and exploitation under capitalism. Some own, others work to enrich those that own. That's all class is. Being frank about the real power differences in our society and our world isn't an ism.
It encourages one to generalize from instances to the group, and from the group to individuals.
Cherry-pick your examples and you smear the group. Invoke the smeared group, and you target the individual.
It's structurally isomorphic to racism and any other -ism.