Because everyone who cares about it is already using some sort of a service like that, and it’s incredibly niche market so it’s not worth the effort to develop and support such projects.

But imagine if Mozilla would have done this before, like capturing the market now proton has

Mozilla should have been doing what Proton is doing. But considering how far Proton has gone I wouldn't be surprised if Proton spun up their own browser at this point.

Not using services funded by Google is pretty high on my personal privacy checklist. Also, they offer some services (like VPN, Relay, Pocket) that usually end up as annoying bloatware in Firefox.

I think password storage is covered by Firefox sync.

Only within the confines of web usage, unless you know something about Firefox Sync injecting passwords into Android apps or environment variables

Also, like it or not, I think Passkeys/WebAuthn is going to be pushed more and more, so without a user focused way to own them, that's another reason not to try and use a browser (any of them!) as a password store