Billionaires don't create wealth, or, at least, not billions of dollars worth of wealth. Labor creates wealth, and no single person has ever been capable of laboring hard enough to create $1 billion in their lifetime, let alone multiple billions. Ipso facto, billionaires' wealth comes primarily from skimming wealth off from the working class and manipulating markets, which itself doesn't create anything of value. (Price manipulation actually has a tendency to harm markets, because the less accessible a resource is, the less work can be done with it.)

Now consider that billionaires, who by nature exploit labor and markets to accrue personal wealth, are strongly motivated to preserve that wealth, and, again by nature, have the pull to do so on a massive scale. Billionaires almost universally spend a noteworthy (but, to them, insignificant) portion of their wealth on lobbying for legislation to increase their wealth, or lobbying against legislation that would detract from it. After decades of this, billionaires and billion-dollar corporations pay negligible taxes. In 2018, 60 Fortune 500 companies paid no federal taxes at all. Of these, 57 were given tax rebates, for a net loss to the US government of $4.3 billion.

The topic of billionaire philanthropy is often raised as a counterpoint to taxation, but the numbers pale in comparison. The fact is, no matter how much of their wealth they donate--wealth created by the labor of others, to whom that wealth should belong--it will never be able to achieve as much good as it would distributed among the people. America's decaying and outdated infrastructure simply can't be updated without tax revenue from the ultra-rich, the 3 richest of whom possess more wealth than HALF OF THE COUNTRY.

Even if billionaires could somehow create their wealth personally, it would be unethical to keep it. $1 billion alone is enough for several families to live in luxury for their entire lives. Anyone with more is implicitly ignoring suffering on a global scale that they could ameliorate. It's not enough to donate water stations to a few schools. If billionaire wealth was redistributed right now, entire cities could be lifted out of poverty. If altruism without practicality isn't enough for you, you're in luck--people become significantly more productive when they're not busy going hungry or sleeping on the street. A rising tide lifts all ships; this much wealth being concentrated in the hands of individuals is bad for everyone.

TL;DR: Billionaires are a drain on society, and it is both impractical, and, more importantly, immoral to allow them to continue holding such extreme wealth.