How do you mean they’ve stolen money from us and “all” their wealth belongs to us?

Wealth is finite.

70 years ago, the wealth that billionaires currently have was in circulation, the rest of us would be paid it, and subsequently spend it, which would then be used to pay others.

That is what the (now mythical) "self sustaining economy" is.

Instead, the same small number of people take huge chunks of wealth out of circulation, they may invest in the stock market, but stock prices increases don't get used to pay salaries, they only translate to money if cashed out.

So in the pursuit of raising stock prices, more and more wealth is removed from circulation

Wealth inequality is when a few remove an oversized amount of wealth from circulation, which is stealing from the rest of us

If someone "earned" a billion dollars and spent it, that wouldn't be such a big deal.

The problem is that wealth being removed from circulation making it literally inaccessible to anyone else unless we unite and take it as a society, done peacefully this is called "taxation".

When that stops being an option, historically there are other options