Because in practice a simple flat tax leaves those on lower or average earnings struglling much more.
You're attempting to cast income tax as some sort of punishment - in the way that it's used on things like cigarettes - this is false and obviously so to anybody.
The principle is those who have benefitted most from this country should contribute most to its infrastructure.
A simple flat rate at 25% would raise less money so that rate would have to go up to say 33% or 35%
So basically you're asking for those with least to pay more so the richest can have more disposable income.
Given that the gap between the richest and poorest has widened even more in recent years that doesn't seem a good idea
http://www.bbc.co.uk/blogs/thereporters/markeaston/weeklyearnings_nep.jpg
I get the picture of a wealthy Etonian holding out a bowl overflowing with truffle oil saying "Please sir can I have some more?"