If you buy them through Google Domains you need your Google account to renew domains, set nameservers etc, which is why these other registrars should be used.
This same argument actually dates back years for not using your web host for your domain registrar, because of contrived problems hindering domain transfers.
Everything I said is true for Google Registry too... and anywhere else Google sells domains. They each should be ensuring domain portability for their respective customers and so far they're not doing that.
Can you clarify what exactly you think we're not doing? (Google Registry, that is.) Transfers are a contractually mandated requirement by ICANN, and we definitely support them.
Also, because of GDPR and WHOIS privacy/proxies, we frequently don't even know who the registrant for a given domain is anyway.
In addition to using another registrar, it's generally a good idea to use single-purpose email accounts not associated with your identity to log in to places like Gandi and Namecheap.
