you have 5 equal companies producing widgets. One of the companies has their building near a freeway, and one day a fully loaded truck crashes through a railing and impacts their building, causing a fire and lots of damage. Sure, they have insurance or whatever, but that company's customers don't care about that, they needed those widgets this week. Luckily one of the other four companies has them in stock and they can fill their order elsewhere. That company they filled the order with now has more capital, perhaps they hire a new employee or buy additional tools or machines. Now they can out-compete the other four, for whatever reason, QC, price, etc. The former company might be unable to keep up with the 3+1 other companies on the same playing field, so may sell to one of the other four, but only the latter company in the above example may have the money available to buy.
This is all simplified and nitpicking what i am saying is futile, because the point is - it doesn't matter how many companies are competing, nothing is in equilibrium, and this doesn't even get in to active sabotage or espionage. A lucky company will buy an unlucky company. Eventually you'll end up with 3 or fewer companies when there used to be dozens or even hundreds.
The end goal of a company is to be like Samsung, General Electric, Sony, etc - make everything, sell everything, own everything. A company being lucky for a few quarters even gets to spend money lobbying for preferential regulations that prevent further competitors from entering the market!
I don't have a solution, because there is a compelling argument to be made for huge companies being able to provide superior quality, price, whatever because of scales of economy, and it just takes one bright lawyer at a multinational to say "but if you break us up it will accelerate climate change because of x, y, z issues we have solved by fact of owning everything!"
Limiting personal wealth to 10-100 million dollars, even "on paper", might prevent this, but that would require global cooperation. I think the upside that things would gradually become more affordable would be a good selling point - if all companies are operating with the understanding the shareholders, owners, etc have a hard ceiling on how much value they can extract from the endeavor...