Because it's all built in. Proton password manager can create a new login for a site. In the same process it also generates a password, creates a new email with a domain that can't be linked back to me, with no setup on my end. Technically I can set up bitwarden to do that but it's a lot of configuration and it will end up using my domain for every email which, even if it can't be linked back to me, all of my email addresses can be linked to each other by merit of using the same domain.

Until there is a major competitor to Proton that solves all of that, there is no alternative.