For example, if you have a $10M net-worth, according to Gabriel Zucman & Thomas Piketty, you get higher returns on average than the little guy. Not sure the exact number, but let's say an ~8% return on average (believe it's higher).
You would make ~$800K a year in "capital gains" and pay $133K in federal taxes. If you earned $500k in income, you'd pay ~$164K in federal taxes. You'd pay another ~$35k+ in states most people live in for about a 39.8% tax rate. Not to mention, your employer is paying another 5% in payroll taxes to employ you. So, really, it's a 44.8% tax.
But if your income comes from "capital gains" instead of working, you'd pay less than half that percentage in tax -- at 16.25%.
Why is there a distinction between "capital gains" and dividends and fixed income? Because only the very wealthy get most of their income from capital gains.
Even people with $1-5M in wealth... they're mostly the people that worked really hard and still got a little lucky with their investments. They're almost all old. They've paid off their house. So... 1) most of their wealth is tied up in their house, and 2) most of what's left over is tied up in fixed income like bonds -- which are taxed as "income" and not "capital gains".