I was like this some years ago. My assumption was that nobody can guess my password, because it is not straightforward. Untill someone explained that if someone new one password, he would know them all. Only then did I start using Keepassxc.

Another factor is inconvenience. On the road, most passwords are out of reach for me now. In a way I prefer it that way, I prefer to not be glued to my phone. As a compromise, there are a few passwords in a note taking app on my phone ;)