Built-in password managers in software like browsers and operating systems are sometimes not as good as dedicated password manager software. The advantage of a built-in password manager is good integration with the software, but it can often be very simple and lack privacy and security features that standalone offerings have.

For example, the password manager in Microsoft Edge doesn’t offer end-to-end encryption at all. Google’s password manager has optional E2EE, and Apple’s offers E2EE by default.

https://www.privacyguides.org/en/passwords/

Why is the built-in password manager disabled?

Use a external password manager, it’s more secure.

https://mullvad.net/en/help/tag/mullvad-browser#102