1) Identity is an email address. If I wanted to rip out Google, or Google kicked me off the platform, all I need to do is add passwords and put a "forgot my password" link and my customers continue business as usual.
2) It's not a google-specific email address. You can create Google accounts for any email address.
3) Google login effectively lets other businesses federate their auth system with ours. When they terminate their ex-employee's @example.com account, the employee loses access to their resources at my company.
I don't think you could get away with this for a consumer company; too many people have strong feelings about FB/G/Apple/whatever. But it's fantastic for B2B.
It's a choice they made, nothing specific to Google or OAuth.
There may be other options if you want to mess with oauth yourself, but this one is pretty near zero effort.