There is no cutoff, but:

• we have increasing brackets to a point, then they stop increasing , which can make upper middle the highest income tax payers
• income tax is only on “regular income” like salary. Wealthier are more likely to get their money as other types of income, taxed at different amounts, or play accounting shenanigans to shelter their money from taxes.
• sales and excise taxes proportionally affect people with less income more, since a much higher proportion of their income goes to necessities
• there’s a tendency to try to correct the regressiveness through greater complication, like my state does, but then are the less well off able to handle the extra paperwork to benefit?

Edit…

• social security and Medicare taxes are only in the first x amount of income so are very regressive, although the payback is also less beneficial to the well off so I’m not sure how to count these