This is because of the functionality called "unattended login". Unattended logins allow us to synchronise accounts and transactions without the user having to log in with their username and password. Unattended logins will usually be allowed for up to 180 days before the user has to present their username and password again.
Unfortunately, there are still some banks that don't support this feature. If you're a customer of one of these banks, then you'll have to use your username and password every time you want to update your accounts and transactions.