Security is all about protecting against risk. If your house is breached (which is probably easier than you realize) and the notebook stolen, you now don’t have your own passwords, and someone has access to all of your accounts. If your house floods, or burns, or the notebook gets misplaced, you lose access to your accounts. If you use your passwords outside of your house, the risk increases. There is nothing secure about storing unencrypted passwords in a notebook (or, certainly, in plaintext on your computer.) I suppose you could argue that all of your passwords are materially complex and unique, you encode them in a way only you know, and you store them in a flood and fireproof safe which can't easily be removed from your house, but I suspect that none of those things are true for most people.
Password managers don’t completely eliminate risk, but properly used, they significantly reduce it while providing significant convenience. In addition to protecting passwords with encryption and MFA (including, optionally, a physical token), they can generate highly complex and strong passwords which most people would not generate on their own. There are other benefits to a password manager, such as breach detection, automated password cycling, passkey support, encrypted credit card and note storage, and syncing across devices, which a paper notebook does not provide.