What at all justifies income taxes (as opposed to sales taxes only)? For those who do not know, in America, the higher your income, the higher the percent of it is spent on taxes. This means that, whatever tax money is used for, those who are more hardworking (or, at least, those who are more skilled) are essentially punished. And, because we tax low-income citizens less, we are making their life quality nearly the same as everyone else, which encourages unemployment. 

If, on the other hand, income taxes were to be eliminated and sales taxes raised significantly, we would be punishing those who spend a lot - essentially, those who abuse their wealth more. (Obviously this is not necessarily always so, but it is more so than if we apply taxes to anything else like income). This, therefore, would encourage philanthropy (and otherwise spending money on things other than buying.)

Note that this can be engineered in such a way that the total tax money the government gains stays the same.

The Question: Why does much of the world still have income taxes? Is there anything (and if so, what?) that makes them better, fairer, or more productive than sales taxes?

*This question is not a copy of the question Are sales taxes regressive? because the linked question is asking specifically whether sales taxes decrease in burden as income increases. My question is explaining that that doesn't matter.
There's a fair amount to unpack in the question, so it might be useful to take it step by step, and consider everything from a more abstract, economic theory perspective.


  ...those who are more hardworking (or, at least, those who are more skilled) are essentially punished....


We should be careful making statements like this for a couple of reasons. First, the assumption may or may not be true, depending on what model of wages you think applies. For example, if wages are largely determined by bargaining, then it isn't clear that wages necessarily directly (or perfectly) correlate with how hard an individual works, or their skill level. Even if we use a non-bargaining wage model, plenty of "unskilled" jobs (for example, auto workers or welders) make more than "skilled" jobs (for example, many teachers). Second, it isn't clear that progressive taxation should be considered "punishing" high income earners. Given that tax brackets are all applied at the margin, earning more only means that (all else equal) your last dollar will be taxed more than your first dollar.


  because we tax low-income citizens less, we are making their life quality nearly the same as everyone else, which encourages unemployment.


While we're flattening the differences between the highest and lowest income earners a bit, I'd hesitate to claim that "we're making the quality of life [of low income earners] nearly the same as high income earners." Given that income still has substantial impacts on quality and duration of life, I'd be hard pressed to agree that we've equalized quality of life.

Beyond that, of course, there's the larger Social Choice problem of "why is somewhat equalizing quality of life a bad thing?" Given that employment and income potential aren't entirely defined by an individual's own effort, but are correlated with many factors that are determined by one's "luck of birth," creating a society that doesn't at least attempt to account for this exogenous "luck" factor seems arbitrary and somewhat cruel.


  ...we would be punishing those who spend a lot - essentially, those who abuse their wealth more.


Again, I'm a bit intrigued by your choice of wording there- why do you consider someone spending income "abusing" it, particularly if we accept your earlier premise that income is tied closely with effort? Isn't spending one's income merely realizing the actual utility potential they earn from doing a job? And if that's the case, why should we discourage spending one's income by substantially raising sales taxes?


  This, therefore, would encourage philanthropy (and otherwise spending money on things other than buying.)


There already exist commonly used mechanisms to encourage philanthropy through taxation: deductions. This method provides a much larger potential incentive to encourage philanthropy, since it effectively makes giving to charity "cheaper" than both spending and holding earned income (since money given isn't counted against your income), as opposed to merely spent income under your proposal. In other words, with an income tax of $\tau$, every dollar I give to charity only costs me $(1-\tau)$, which is the amount I would keep if I took the dollar as opposed to donate it. If there's only a sales tax, then the cost is the full dollar.

So, getting to the crux of the question- there are several reasons the world currently has income taxes. First, there's a distributional concern: the sales tax that would be required to offset income taxes would be quite high, and would cause significant hardship for poorer families. Moreover, only having a sales tax can lead to incentives for wealthier individuals to horde wealth as opposed to spend it, which actually leads to significant economic distortions, and adverse economic outcomes (for example, a fair amount of recent research suggests that too high levels of wealth inequality can lead to slower overall economic growth). As to whether there are fairness implications, I'd point out that the regressive nature of sales taxes are important on this point. Given that a comparatively larger burden of the tax would fall on poorer individuals (since they spend higher proportions of their income), eliminating the income tax in favor of a consumption tax would have distributional consequences.

I don't mean to sound nit-picky or like I'm dismissing your question out of hand, for the record. There is an important debate to be had for any society regarding how its citizens should shoulder the combined tax load. However, I think most of the debate occurs "inside" of your assumptions made in the question, and only by closely examining those can the debate be meaningful.