Because, in liberal (small-l, read: the West) democracies, the notion that taxation policy should be used to encourage productive behaviours and discourage unproductive ones is deeply-enshrined. Thus do we give tax breaks to people who start companies, create jobs, or invest in same, and further penalize with a luxury tax those who just go and buy a big fuck-off yacht. We give tax breaks to local farmers, but tax cigarettes and booze.

If we had the kind of tax structure you propose, we'd equally reward those who invest their money in growing companies, and those who blow it on hookers and blow. Some libertarians would claim this as a just tax system, and that the free market would determine how and where people invest, but this is not an idea that has much purchase among economists, social scientists, or the political elite.