Why can't we amend our 2020 tax return that has already been filed to get the unemployment benefit when others like H

You don't need to. 

The IRS will adjust the return you already filed and send you a refund if applicable.

The IRS does not want to deal with amended returns for that, they find it easier to simply correct the returns already filed. 

You generally do not need to amend for Federal.

However, taxpayers would have to file an amended return if they did not originally claim the EITC or other credits, but now are eligible because the [unemployment comp] exclusion changed their income.  These taxpayers may want to review their state tax returns as well.

IRS to recalculate taxes on unemployment benefits; refunds to start in May | Internal Revenue Service

See these re State:

How is my state taxing unemployment income? (intuit.com)

Do I need to amend my state return to update my un... (intuit.com)