Personally, I like if not everything is going through my phone.

It can be distracting. You have social media, messages and apps on the same device you need to login. Already makes it harder to put away the phone for a while.

It can never be "cold": You have your phone always with you, which means the risk of it being stolen or lost is higher than with something you can keep in your desk drawer for 99% of the time.

It's a single point of failure: If you should lose it, you potentially have a huge problem: You're right that a thief couldn't make much use of it and we have sophisticated tech to remote-lock and locate phones now. But that won't help you if you need your phone to log into your Google or Apple account to access that tech in the first place.

(Yes, I know there alternative ways to regain access to Apple or Google accounts, but those all rely on you having something... not on your phone)

I don't really own the phone. I know it's paranoid, but I don't like the idea of some update being able to change the way authentication is done. (Even if this is improbable in practice) At least FIDO2 firmware has a fixed interface.