Password managers have a few problems. Sometimes they just don't work well (e.g. banking sites that partially obfuscate your username really confuse them). Sometimes websites make it hard to copy and paste passwords. Apps don't always work with password managers, requiring manually retrieving the password. But mainly, password managers are inherently a bit scary as a single point of failure. You might say that OAuth has the same problem, but I think it's better because no passwords are stored anywhere (even if encrypted), and I use a hardware 2FA key for my preferred OAuth account.

I think the concern about "data grabs" is a bit overblown. I don't believe the OAuth logins that inform you of what information is being shared is lying, and if I don't like what is being shared I won't use it.