Let's say you make $50K every year after tax. You spend none of it, and it all goes under your mattress.

In 20 years, you'll have a million dollars.

To be a billionaire, you need to make $5 million a year for 200* years, and spend none of it.

And we're not even talking about interest. If you have $1M in investments, with an extremely modest 3% interest rate, that's a passive income of $30K a year, taxed only under capital gains, which is far less than how much you'd be taxed if it were regular income. (Wealthy people often bring in 10%+ interest from their investments.)

$1 billion is an obscene amount of money, even without the interest and tax issues.

*Typo. Not 20 years, 200 years.