What makes the built-in database easier to attack than a separate one?
For performance reasons, early versions weren’t even encrypted, and later versions were encrypted with easily-cracked encryption. Most malware broke the encryption on the password DB using the user’s own hardware resources before it was even uploaded to the mothership. And not everyone has skookum GPUs, so that bit was particularly damning.
Plus, the built-in password managers operated within the context of the browser to do things like auto-fill, which meant only the browser needed to be compromised in order to expose the password DB.
Modern password managers like BitWarden can be configured with truly crazy levels of encryption, such that it would be very difficult for even nation-states to break into a backed-up or offline vault.