Its our country, our world. You don't live on it alone; we live here together. People who are very rich don't earn proportionally on how hard they work. Its the people who earn very little who do the hard, tough labor.
Money is a societal construct and as such it belongs to exactly the person society says it belongs to. That may not be the billionaire depending on the society.
Everything in society is a social construct, implicit or explicit. Right now the social construct that we have is that you get to keep the money you worked for after paying some taxes.
I do wonder if you could make something like the wealth tax work in the United States.
Well, it is their money.