@Mandy3rn because a dead person didn't fill out a tax form incorrectly. And from the 2009 experience, most families returned the money as the 'right thing to do', even though not required.
in the case above, the TAXPAYER FILLED OUT the tax form incorrectly. The money was ill-gotten and violates the statement that all of us sign at the bottom of the tax form. You are liable for penalties, interest - and in the most egregious cases - jail time - for filing an incorrect form and not correcting it once it is known to be incorrect. Failure to return money that is ill-gotten is tax fraud!
That is why.
