Because that is how the law says it is to be handled?
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Perhaps it would help to review what is really happening:
The funds were paid out as Advanced payment against a projection. The projection used 2018 or 2019 tax returns. But 2020 is the Actuals. You use the 2020 return to reconcile what a person is entitled to, against what they got.
You might want to bookmark these links and read the IRS guidance.
One for each EIP.
"Level Up" is a gaming function, not a real life function.