I haven't used the browsers built-in password manager for years, so I don't know what features they have, but I find it hard to believe that they can provide the same functionality as a dedicated password manager.
Some of the top features of dedicated password managers include:
* Generating random passwords/passphrases (this is pretty basic)
* Storing and generating two-factor authentication codes (TOTP)
* Filling out passwords into mobile apps as well as websites
* Storing security questions, back up codes, any other site specific data that needs to be secure
* Storing credit card information
* Platform agnostic syncing
* Sharing passwords with friends, co-workers, or family
* Weak password checking / HIBP integration
I'm sure that the browser password manager can do some of these things, but I doubt it can really do all of them.