People who make money by investing. In the USA, the top 1% earn their income through investments, usually the purchase and sale of stocks. These are not taxed the same as regular income because they made the argument that you can’t really tax unrealized gains on investments that are sold, and it takes a while for the gains to actually materialize. Also, they tend to store their money, their liquid assets, in countries with looser tax laws, called tax havens. Much of their net worths are tied up in investments. Businesses, homes, art, classic vehicles, precious metals futures, oil futures, boats, etc.

Assessing the value of all of that is a chore, and they also pay lobbyists to keep the IRS defanged so that they don’t have the resources needed to go after the 1%. And don’t get me started on how much more speculative the stock market has become. Investors buy stocks, not on the expected dividends they’ll receive as a share of the profits of the business, but on their ability to flip the stock and sell it at a higher price to another investor, who is only buying because they anticipate flipping the stock. It’s like if a whole neighborhood of single family homes gets bought up buy a few house flippers, who make renovations, then put the houses up for sale, and sell to new flippers, who are only buying so they can make further renovations, increasing the value of the property again to sell to yet another flipper, ad nauseam.