Apple Mail in Big Sur

It must think the mailboxes are already in view.

It seems that way. Try this: Quit the Mail app if it is open. Then, press and hold a Shift key while launching Mail from Applications or the Dock.

That action opens Mail in windowed mode (not full screen) with no message selected.

Release the Shift key when the Mail app opens. See if that causes the mailbox list to repopulate.

Assuming those are local Mailboxes there ought to be a heading labeled "On My Mac" in Mail's mailbox list. To show that list, View > Show Mailbox List or use the key chord ⌘ ⇧ M

Then, you have to position your cursor over "On My Mac" to show its "reveal triangle". Click it to expand the list of local Mailboxes.

My pleasure. Glad to help.