Te State is the biggest cost and the biggest spender within the State. They take the largest proportion of the cash within the economy and spend it. If they are inefficient then the country is inefficient. Inefficiency is the biggest driver of excess cost.
Interestingly the Dept of Finance argue that decreasing it would encourage more aggressive tax planning to shift what should be taxed as income into the CGT regime, whereas keeping income tax rates and CGT rates similar makes the risk of such activity less attractive.