But the reason is more because on the high end of income liberals (well, at least myself) believe a lot of that income is not because someone is doing a great job, but because it's far easier to make money with money than it is to make money without money. Ignoring taxes, inflation, and choosing a low investment interest rate (5%) to make up for those assumptions, consider this scenario. You have no money, but you have a job and are able to save $10,000 a year. You invest at 5%. You'll have your first million dollars in the bank (well, investments) in 36.72 years. Your second million only takes an additional 12.43 years, about 1/3rd the time. Your third million only takes an additional 7.68 years. Your fourth million 5.57 years. You see, as your wealth goes up it's easier and easier to earn that million.
Is the person with 3 million a better producer? I guess you could say they are, they're able to fund more businesses, etc... than the person who is investing their initial $10,000 in this scenario. Are they working any harder? No, they don't have to be. The interesting question, what happens when they die?
Give that 4 million to their kid, assume no estate tax, and the kid can turn that 4 million into 5 in 4.38 years. What it took his parent to do in 37 years he has done in 4.
Why people want to have progressive taxes, estate taxes, that sort of thing is because it's not perceived as fair that it's so much easier to make money, when you already have money. We like to believe if we work hard and have awesome skills we can climb the financial ladder. But it's a very difficult ladder to climb when you are at a financial disadvantage to others. Because it can take you 37 years to do what someone with a head start can do in 4 years, equal skill, equally good with money, equal in everything other than a starting point. (The actual years/difference will depend upon where you put inflation, the investment interest rate, taxes, etc.)