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