What is a corporation but people? Companies are groups of people. They're one or more owners, some managers, possibly some staff. If you take away all the people, then there's nothing there.
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.
Assets, mind share, etc. All the things that get sold off in bankruptcy proceedings.
A railroad company also has a huge amount of steel roads with cross country right of ways. You could fire everyone, and it'd still be extremely valuable. It might even be more valuable.
Even in asset-lite software land, "Atari" the name still has a lot of value, even though none of the people are there anymore. They own the right to control the things that the neurons in millions or billions of people's brains are tied to, that their habits tie them to.
Twitter, you can fire almost everyone, make the online square less pleasant for a lot of the people, the network effects truck on. People continue to use it even as they hate using it, because most everyone else is.
A corporation is a liability and pseudonym shield.
A corporation having contribution rights multiplies the otherwise imposed contribution limits on the underlying humans.
A corporation allows tax planning opportunities not afforded to the bottom 90-ish percent of the country (such as shifting income to alternate years to avoid progressive taxation, and utilizing expenses to lower that income in ways a W2 cannot)
Corporations are granted benefits that people do not get, and CU disturbs the balance of power.
Legally, the corporation itself is similar to a person and has rights unto itself, as stupid as that sounds. Those rights are different from those the individuals who run the company have.
Corporations no.