This is a serious question. I'm rereading "The First Modern Economy: Success, Failure, and Perseverance in the Dutch Economy, 1500-1815." The authors take some time to discuss the rentiers of the 18th century, who were criticized both by contemporaries and historians for "a tendency on the part of capital... to prefer speculative trading... and investment in money lending and insurance, to the toil and hazards of foreign trade." (quote from Violet Barbour). The authors make a case that in part this was a natural reaction to a less-friendly trade environment --- mercantilist barriers were going up all over Europe --- but also a consequence of having more money than they really knew what to do with:
The vast accumulations of capital in the hands of the eighteenth-century capitalists placed them (ironically) in a position of dependence vis-a-vis national states. Only the states, with their taxing power, could hope to absorb such sums and make regular interest payments. The fact that governments sought to borrow almost exclusively to wage war guaranteed that Dutch capital would do little to stimulate economic growth directly; but neither the immediate economic possibilities nor the grandiose development projects that we, in retrospect, may be capable of imagining could have absorbed a large portion of the late eighteenth-century stock of financial capital. In the circumstances of the times, governments held a monopsonistic position in the capital markets.
(I think this is not quite on point because the Dutch did have some pretty grandiose empire-building designs of their own, which absorbed quite a bit of capital, they just didn't work out.)
The same criticism persists today, about too much money sloshing around in financial instruments instead of the "real economy." But, absent some grand redistributionary scheme, what exactly should the ultra-rich be doing with their money? (We have seen how even voluntary redistribution provokes acerbic backlash on the means and goals.)
Loaning it to the government is still a popular choice, and like the 17th and 18th century, it goes mainly to fund wars rather than development projects.
Starting a business takes lots of money --- and that's how at least a couple billionaires I know choose to use their money --- but, if the business is successful, only compounds the problem. And presumably we don't want capital going to unsuccessful businesses.
There's arguably a limit to even how much the startup ecosystem can fruitfully absorb. Small companies just don't need all that much capital. Plus an excess of cash leads to self-fulfilling prophecies in which success is measured by the ability to raise more cash.
Plunging all your cash into a 2-billion-dollar iron mine increasingly seems like a bad idea. Buying somebody else's company just shifts the problem around.
So where *should* all that capital be going? Is there even an answer? Clean energy? Our best answers don't seem all that different from the 18th century Dutch:
1. War and interest service on war
2. Naked speculation
4. World domination (aka Elon Musk, although in his case "space" might be more apropos)