The flat tax [applied through a sales tax] is a much more efficient tax. aka... it's cheaper to collect and control evasion.
In order for it to work you have to impose it equally on all economic actors, people and corporations equally... sure, they won't get taxed on revenue, but every time they spend money they'll get taxed.
if the gov's true intention was to maximize profits from taxes, it'll consider the flat tax.
Sometimes you obtain a larger revenue by lowering taxes. If you set taxes to 0% your revenue = $0. if you set taxes to 100% your revenue ='s $0 [all economic activity goes into evasion and any economic activity that doesn't evade, goes out of business].
So, you need to set taxes to the % that gives you the highest revenue...
Here's were the flat tax comes in handy:
With the sales tax [or an aggregated value tax, which works in pretty much the same way], you simply need to adjust one tax to try and optimize your revenue, and supervision is at it's cheapest.
Regarding the acountants, they'll still be needed inside corporations, for internal controls and for making reports that are required by Exchanges and Banks [when you ask for a loan] and other means of financing...