If all the taxes were combined into the a single tax aka the PAP tax, Sporeans would know how heavily they are being taxed.  Inconvenience to the public is not a concern to the PAP.  They can simply impose fines for  any mistakes made by the public, extra $$ for the PAP.
By separating the various taxes:  COE, ERP, CPF, Radio/TV tax, public transport tax, water levy, .... the PAP can maintain the illusion of low taxes.  They can also tweak the taxes by increasing the various taxes upwards 
