TurboTax starts out your return by assuming that you did not receive any stimulus payments. So your refund starts out not at zero, but at the amount of the two stimulus payments.
When you tell TurboTax that you have already received the stimulus payments, then those amounts are deducted from your refund. Otherwise your refund would be too high and you would have to pay it back or have your refund held up while the IRS investigated the overpayment.
