The question is not "why wouldn't I?", the question is "why are old login informations still relevant?". The old login informations might have been changed because they got somehow compromised: they should not be valid anymore anywhere, but actually they are.It is poor password management. I hadn't done for my Apple ID and password because I follow a formula that is easy to remember, but I do it for any account where I change my user ID and password. Why wouldn't I? I keep any old user ID and password for Microsoft account, Google Play Store, banks, etc. Simply because I do change my user ID and password as it is good practice, and because it is possible to forget a user name and password when you change it, so it's a good idea to keep it. As I said, the only one I haven't got written down is my Apple because I follow a specific formula. Now, after this, I've added it to my password apps.
And the person who taught me this? My mother, who is not a tecchy person but is OCD about keeping records.
The problem is not only that they need to be used to unlock the device (which could be counter-intuitive and potentially an hassle), the problem is that they can be used to do such a thing at all (which could be plain simply a security risk).