Importing archived mailboxes to Apple Mail — structure of folders

I have moved to a new MBP (M1, Big Sur), and I am recovering my old documents and software slowly, piece by piece, through new installations and/or TimeMachine backups (I did not do a full restore from TimeMachine).

Now I'm trying to import some "local" (archived, "on my Mac") mailboxes that I used to have in my previous Mac's Apple Mail. I wonder why, after importing, they have their whole "structure" visible, which was not the case before.

Here I explain the steps I followed and the result I obtain.

First, I located the archived, "local" mailboxes in my TimeMachine backup (on an external hard drive) under ./Library/Mail/V7:

Here are the folders I am interested in recovering: the ones named 2015, 2016, etc...

Then I go into my Apple Mail, choose from the Menu "File">"Import Mailboxes...", and then I face this option:

I choose "Apple Mail" (albeit the folders in the library have a suffix .mbox). Next, after I choose the "2015.mbox" folder from the TimeMachine, I am shown this option:

I leave everything checked as shown, and choose "Continue". The result is:

Sure, the messages are all in the folder Import-2/2015. But on my previous computer, I was not shown all the "ugly innards" of the 2015 folder, no subfolders called "99D131..." etc.

Why does this (previously hidden) structure show up when I import the messages from the old Library (via TimeMachine)?