What demands do people with very large amounts of money make on the country and its resources as opposed to people with very small amounts of money, Denise? I once had relatives who owned a meat packing plant. They employed a large number of people and they were personally very nice people and I loved them a lot. I still believe that having them pay the same, say, 15% in taxes would have been wrong. Their trucks caused a lot more wear on roads, for example, which meant the town was constantly repairing roads leading to and from their plant, and when you dump large amounts of animal waste down the sewers, which are meant for everybody, that makes the processing of that waste much more difficult and expensive. When you add to that the fact that the meat processing plant's solution was to flush the drains with large amounts of acid on a regular basis, you have a pollution problem on top of the other problems, and a much accelerated wear problem on the sewers which my relative's meat packing plant certainly didn't want to pay. They wanted to take money in, not pay it out.
The fifteen percent business wouldn't have even come close. You can see this being replayed across the country for many businesses.
On the other end of the spectrum, you have people whose incomes place them in extreme poverty, and to take 15% of that would put them in danger, or put their kids in danger. In fact, though it seems fair, a flat tax turns out to be a regressive tax in which the burden falls most heavily on the poor, who can least afford it. That is why it is unfair, and that is why it is almost always you will find the wealthy supporting it; it breaks heavily in their favor and it punishes the poor for being poor.
In fact many of the poor work as hard as the rich. They're simply nowhere nearly as well rewarded.
I suspect you've had people go over this material with you before, Denise, but I thought I'd give it a try. I hope I was at least a little helpful.