Why do we need a Chartered Accountant to pay tax?

In my opinion the process and and calculations should be so simple that even a layman should be able to pay tax without any expert advice. Flat tax rate for all can be a good option to make things simple.

Think about people with less earnings

People who are earning less need some relaxation on tax. That is the reason we have slabs for different tax rates. There would be reasons for other complexities as well. Not sure though.