It's not. Keychain is not synced across devices. iCloud keychain, a separate service, can sync parts of keychain.
iCloud keychain is a perfectly reasonable (and as a UI, probably better than anything else) password manager iff you use Safari as your main browser and all your other devices are Apple devices.
OK, that’s something I didn’t know, and I may be missing something from such a silly name overlap. However, I do have items in Keychain Access.app which are from iCloud. What gives?
Keychain Access lets you view your local keychain (i.e. your device's secure trust store). If you have iCloud keychain turned on then certain parts of your keychain will be synced across devices so you'll be able to find, say, a web password you generated on your phone on in your Mac's keychain (via Keychain Access and otherwise). The terminology is a bit confusing, that much is true.
If you can live within the constraints of iCloud keychain (the Safari/Apple devices thing, don't need stuff like 'team sharing, etc) it's arguably a better solution than 1Password.
iCloud keychain is a perfectly reasonable (and as a UI, probably better than anything else) password manager iff you use Safari as your main browser and all your other devices are Apple devices.