Because most rich people don’t feel like they’re rich.
Most people who have a decent, salaried job in the USA have nothing to complain about, from a material standpoint. Healthcare, housing, food, retirement, they’re all taken care of. Where this falls is up for debate, but even in one of the three highest cost of living areas in the USA I personally consume <15k, post tax. That’s as a single guy in his late 20s and doesn’t include the healthcare benefits I receive as part of my compensation package. So let’s generously say that a single person making 40k has reached saturation in consuming necessities.
At that point, why should they try to make more? It comes down to status, and status is inherently relative. You end up comparing yourself to your peer group. And the general tendency is for people to spend most of their time with people of similar means, with some outliers in either direction. You usually note the person making the most that you know, though, instead of taking a more objective viewpoint that you’re in a privileged class of the most privileged nation in the world in the most privileged era of history.
Wealth is distributed through according to a power law, too, which makes things even worse. Someone making 20k a year won’t know many people making 100k/year, while someone making 100k/year is likely to know many making 500k/year and a couple making a million annually. So by becoming richer and expanding your peer group, you psychologically feel even more squeezed. To keep up with everyone else, you’ve suddenly have to send your kid to the best private elementary school that costs 40k/year, take vacations in faraway lands, and get a nice five bedroom in the Upper East Side.
So, say you’re making 250k/year, and there’s a proposed hike in the income tax that would cost you an additional 20k. Although you’d expect it to hit everyone in your peer group equally, in practice the people making a million a year are likely to end up only having to pay an additional 40k instead of a commensurate 80k. You’re further and further away from reaching your (nearly unattainable) goal of status supremacy.
Unless you’re Warren Buffett or Bill Gates, and everyone already knows you’re definitely richer than them. Then taxes are hunky dory.