Sure, interest after 6 years can accumulate into a nice penalty sum on top of what is owed. If IRS ends up owing you, and they discover it 6 years later, would they send you the refund with an interest as well? I am guessing they won't.
They also sent me some form the year after that so I could declare the couple dollars interest I got from the IRS as income.
This is probably just a practical matter, because accounting for interest during the tax year would be complex/impossible because you would need to know when the various amounts of withholding exceeded the amount owed. I'm not even sure there is a single consistent way of doing this.