On a practical level, individuals with greater income or wealth can pay more in taxes than those with lower incomes or less wealth.
For example, the first $15k an individual earns will afford them things like food and shelter, without which they would likely contribute to the destabilization of society. Take too much of that $15k away and you greatly diminish that person's quality of life and ability to be a productive member of society. On the other extreme, look at the last $15k earned by someone with a $150k income. You can take away all of that last $15k and it will have a comparatively minor impact on their life and lifestyle.