If you have messages set to be marked as read after say 5 seconds, you will end up in a cascade with the message being marked as read after 5 seconds and then removed, then the next one automatically being marked as read after 5 seconds and then removed and so on until there are no messages left. What a shock if you stepped away to receive a parcel and come back to find all your messages are read and nowhere to be found.
So we keep the read messages in display until you move to another folder. When you come back to the unread folder later, only unread messages will be displayed.