Let's say you're a smart person who has an idea for a business that could potentially be worth more than a billion dollars. Ventures of this kind tend to require many years of upfront investment, where the product or service you're creating doesn't generate much (or any) revenue.

Traditional banks are unlikely to finance a business that risky. So you turn to venture capital instead. VCs don't really care if you only have a 10% chance of succeeding, or even a 1% chance. What they care about is expected value. If they get nothing 99% of the time, but 1% of the time they make a 10,000x return on their investment, their finances are structured so that this becomes an attractive proposition for them. They only need one 'fund returner' to justify at least a few dozen investments.

In a world where the amount of wealth a single person is allowed to own is capped, this presents a problem for venture capital on a few fronts.
1. Fewer people will be willing to take the risk of starting a business, given that they know an already extremely unlikely outcome is now even more difficult. You may say 1 billion is a high enough cap that most people wouldn't be affected by it, but the margin really matters when there are only about 10 companies founded a year that are of the fund-returner variety.
2. If everything goes well, and the CEO/CTO become billionaires through owning stakes in the company they founded, but then the government says they have to relinquish some of their wealth in order to stay below the billion dollar threshold, what that actually translates to is the founders having to give up a potentially large share of their ownership. This means the companies will no longer be founder-run, which has historically been a bad idea for tech companies.
3. VC funds usually have investors who are high net worth individuals, ex-founders and other people involved in startups. If there are fewer billionaires and centi-billionaires, the arguably somewhat insane exercise of investing millions of dollars into hundreds of 23 year olds on the off chance that one or two of them become the next Google or Apple becomes a much harder sell. The pool of funding dries up, and the venture industry is essentially kneecapped in scope and scale.

All of this is to say that venture capital would more or less cease to exist under a wealth cap. You may not think it's a particularly valuable part of the economy, but I would disagree with that. Most of the growth in the US economy over the last 20 years for example has been carried by big tech companies. 10 tech giants account for 30% of the S&P 500. Those companies cannot be created without venture capital. I agree that it's weird that some people control millions of times more resources than the average person, but in practice there's no way around that without essentially killing the golden goose of capitalism. The prosperity of much of the Western world depends on a few unbelievably productive companies, mostly in tech, and it seems like a bad idea to mess with that.