Why can't I enter a future date for a direct payment on a balance due?

Because the IRS made that rule.

If the taxpayer wants to pay at some point in the future, the taxpayer needs to make that payment (such a going to IRS Direct Pay).

If they are on extension, go to screen 9 and input a 1 in the first item. Complete the rest of Screen 9 to match the extension mailed.

If they are not on extension, just leave the payment date blank and IRS will withdraw the money when they process the return. Press F1 at that entry in screen  3 and you will get this info:

Electronic payment date [O]

The program uses a default electronic payment date of April 15, 2021 for returns filed before and on April 15, 2021. For returns file after April 15, 2021, the electronic payment date will be the transmission date. Enter the date, in mm/dd/yy format, you want the debit of the electronic payment to take place before April 15, 2021. The program will not allow you to enter a date later than April 15, 2021.