Because it is about maintaining control.

They try to prevent competition from coming up the way they did.

Communism places a small group of powerful men in control of a nation-state's capital and power. First, please note that "capital" does not merely mean "money" as many contemporary thinkers posit. Capital implies anything that can be used to create value, be it labor, materiel, tools, ideas, investments, etc.. Value can easily be thought of as "money", but the two are not quite synonymous. Next, controlling a nation-state's power implies both "soft" and "hard" power. For examples. the State Department is "soft power" and is representative of the "carrot" of diplomacy. It offers aid and assistance (and with it, the implicit expectation that you will play nice so we can gain more capital). You scratch our back, we'll scratch yours. So long as everyone gets what they want at a price they're willing to pay, this all works out. The DoD is "hard power." It is the stick which will rain down destruction and ruination (and will usher in the rule of people who will accept the offers and expectations of "soft power") if you don't toe the line. It is the implicit threat of "play nice or we will kick your teeth down your goddamned throat, then take your lunch money."

If you are already powerful and well capitalized, removing competitors enables you to claim a larger piece of the pie, reduces competition, and lets you exclude others from making any sort of gains against you. Marx listed ten points of communism, and all ten have at least had their foundations laid in the US to some extent or another. We are closer to "Das Kapital" than "The Wealth of Nations" and the push towards Marx's "inevitable" system will only grow.

Neutral answer:

The more detached you become from society - which wealth tends to do to those that have it - the more you see society as a cost-benefit analysis, without an emotional or entropic input. So Communism starts to look like the best way to do things - assuming the wealthy person in question becomes one of the Commisariat.

Cynical Answer:

They envision themselves as one of the Commisariat off the bat, if for no other reason than they had the intelligence, insight and presence of mind to understand this was the correct path to take. And that would be a lifetime position, because economic mobility in a Communist state is non-existent.

Cause it's communism for thee not for me. It's easy to spend money from someone else pocket.

As Tony Montana once said...

First you get the money, then you get the power.

communism makes everyone poor. if they kill capitalism a challenger will never be able to rise against them.

Wall Street funded the Bolsheviks.

Small businesses get rich through capitalism. Large corporations stay rich through the power of the state. The state can fine them, break them up, or pass laws favorable to them by outlawing competition or increasing the barrier to entry. Look at the antitrust lawsuit against Microsoft for bundling Internet Explorer with Windows. Microsoft didn't pay enough attention to the political game back then and lost out badly. Since then it has learned.

Virtue signaling is a way to appease politicians to make it more difficult for them to go after your assets.

The answer lies with r/K theory.

r selected rabbits (aka liberals and communists) tend to be on the whole more artistic and innovative as compared to conservatives. As such liberals (r-selected) are great at invention and bad at long term management. Which is why you often see them turn communist and SJW converged cause they are just being true to themselves. They were innovative, became lucky in their success and because they are liberals since they don't have a mindset for long term critical thinking and implications they fail spectacularly.

because they think their money will protect them from communism.

There was never so much wealth in so few hands than under communism.

White guilt and good old fashion self loathing.

I think the people running those companies were always commies, but they were only paid attention once they became rich. Most of the tech companies come from California.

i'm in the beginning phases of a hypothesis about super saturation of resources... this pattern fits in every single place communism/socialism has been attempted.