This is ultimately about power. Democracy is built on the idea that people are equal and have equal rights. This is only sustainable if the power differential

This is ultimately about power. Democracy is built on the idea that people are equal and have equal rights. This is only sustainable if the power differential between people isn't too large. Corporations consist of people, yes, but the power to control where the efforts of these people are directed is concentrated on very few people, giving those people a large amount of power -- so large that without any checks on their influence on government, it undermines democracy. It's the same argument for why super-wealthy individuals are so problematic. They're literally destroying the fabric of democracy through the power imbalance afforded by their wealth.