What do you call someone who accumulates an excessive amount of something, more than they would ever reasonably want or need, and is highly resistant to decreasing their supply of it? A hoarder. And that's what billionaires are: hoarders of money. On the surface, this sounds preposterous; after all, how can one ever have TOO MUCH money? Such an assumption is largely due to human civilization developing currency, replacing commodities, livestock, and precious metals with money as our main store of value. But money isn't inherently valuable, and so it doesn't get to avoid being a “hoardable” item.
Think about this: the average person makes $1.7 million throughout their entire life. Yes, that's every dollar in every paycheck you get from cradle to coffin, and at a minimum, a billionaire makes over 500 times more. With entire lifetimes of wealth in their hands before they've even died, they have more than enough to keep themselves happy. And yet they keep accumulating more. They and generations of their progeny can live their lives in complete luxury, by it's not enough. They could pay their employees a living wage, uplift those in need out of their circumstances, or make the world a better place with their massive supplies of value. They could do all of this while living the exact same lifestyle, noticing their expended fortunes only when looking at the numbers in their bank accounts. But those numbers are exactly why they live their lives the way they do. Having the highest standard of living in all of human history is meaningless to them when there's still money in the world that isn't theirs, valueless value free to be accumulated.
Do you know why most people can never be billionaires? It has not as much to do with opportunity, intellect, work ethic, or education as it does with personality. Every generation, there are hundreds of thousands of people who meet all of the other qualifications, yet never have a chance at billionairehood. What makes these otherwise successful people ineligible? They like money because it gets them things that they want. They know that the point of money is to more effectively pay for the goods and services you want when you want them. You may not be able to buy happiness, but people who have more money to buy things with tend to be happier (to an extent). Poorer people also use money to buy things they want, though that “want” is usually slanted more towards necessity than luxury. Billionaires, however, are different. With enough money to buy anything that can be bought, they can't like money because it gets them things that they want; otherwise, they'd have stopped trying to get richer long ago. Instead, they like money for the money itself.
Look at it this way: most people have a relationship with money of “have money -> pay for whatever I want”. The ineligible rich also follow this relationship, though the first step is a lot easier for them. In contrast, billionaires have a relationship with money of “have money -> get more money”. The key difference with these two relationships is their durations. Paying for whatever you want when you have money only occurs for as long as you want things. When you don't, the relationship ceases, and when you do, you choose to restart it. The billionaire relationship with money, however, gives no choice to stop. Once they get more money, they “have money” again, and the goal to get more money returns, causing a feedback loop. It's an endless cycle of meaningless greed. Most people, including the ineligible rich, will never get close to billionairehood because they lack this drive to hoard money.
But why can't our society simply ignore billionaires like we do millions of other mentally ill people? Because they have an impact on us. In their endless mission to keep and accrue more and more money, they hoard value from the rest of the world, value that could do so much for so many. Worse yet, we celebrate them for it because we don't understand what's wrong with what they do. Capitalism only works when money is being shared around. You pay someone for something, they pay some more people with that money, and the cycle goes on. However, that circulation of value ends the moment it reaches a billionaire. They conserve the maximum amount of every penny that comes their way, and that money stays in their bank accounts, never “trickling down”. Capitalism has always been imperfect, but the existence of billionaires makes it outright broken. We all have to accept that, even in a capitalist society, enough is enough. We have allowed a system created to incentive hard work and risk-taking devolve into one which champions a tiny caste of mentally ill people at the expense of everyone else, and the only way to fix things is to stop their hoarding.
tl;dr: Billionaires are money hoarders because they collect more money than they could ever want to buy stuff with, and that's bad.