Because if you don't use one, you'll almost certainly instead either reuse passwords across sites, store passwords insecurely, or choose weak passwords.
There is not a lot of value in trying to attack a single person's password file, but the value is multiplied by the number of users when using a centralized platform.
That is too hard or tedious for most people. Syncing is a pain, and doesn't autofill on websites or in apps.
Which you almost certainly access with some kind of software, yes?
A password manager is just a piece of software that stores passwords in encrypted files, and makes it more convenient to edit them, associate them with a website, use them to log in, check them against known breaches, etc.
Enpass isn't open source but it's open... implementation details? There isn't a formal spec or standard but they were very forthcoming about how their encrypted SQLite implementation works and there are now open source third-party CLIs for it. https://github.com/hazcod/enpass-cli