Don't know why*, but here's what you do about it:
In regards to the stimulus payment, when you file your 2020 income tax return, you will be asked for information about the amount of the stimulus payment you received, and if it is less than you are entitled to, you will be given a credit on your 2020 tax return.
"In essence, the stimulus check acts as an advance of your 2020 income tax refund. This means when you prepare your 2020 income tax return, there will be a line to include the section 6428 credit (line 30 on the 2020 form 1040). The credit on your 2020 return is subtracted by any amount received as a stimulus check in 2020. If the amount you received as a stimulus check is less than the credit you are due, the difference will be included as part of your 2020 refund. If you have been overpaid by receiving the stimulus check, however, you will not be required to return any excess amount".
* The most likely reason: Under the CARES Act, if you are claimed, or qualify to be claimed, as a dependent on someone else’s return you cannot receive a stimulus check, in 2020. If you qualified as a dependent for 2019, but will not be for 2020, you will most likely get it in 2021, when you file a 2020 tax return.
