Since my filing was accepted 2-21-2021 but never received, why does the IRS expect their money immediately if I now owe on my amended 2020 return?

If the original refund was not yet received since it has not been processed you should NOT have filed the amended return. 

If you did then you will be in a long wait ... amended returns normally take 4 to 6 months to process AFTER the original return has processed.   

And review the 1040X ...  do you actually owe ?  Did you change line 18 to reflect the original refund that you did not get ?  

Column A should have the figures from the original return, Column C the corrected figures and Column B the differences between the other 2 which needs an explanation on page 2 of the form 1040X.

If you completed it correctly you will see an entry on either line 16 OR 18:

On line 16 should be the amount you paid with your original return. 

On line 18 should be the amount of your original refund you received.

Then you will see your extra refund on line 22 OR  the new balance due you need to pay on line 20.