It has been this way for many, many years so I think the specifics of why the decision was made are likely lost in time. But I can give you what I think is likely the reason based on my understanding of why Trello does some other things the way it does.
Trello, in general, tries not to assume anything. Moving a card to a “Done” list doesn’t mark the due date complete. Nor does marking the due date complete finish checklist items. You have to explicitly tell Trello not to show completed cards. There are very few cascading actions that a user can take in Trello. My guess is that the intention was to continue maintaining that. The state of a list shouldn’t update the state of a card.
There are also some interesting questions around unarchiving lists. If you unarchive a list, do you expect the cards to be unarchived as well? All of the cards in the list? Or only the cards that were archived when the list was archived?
But it could also be the case that it just ended up being implemented that way and no one really thought more about it.