It's just not a replacement for TOTP and password entirely. It's a shortcut, a super-cookie that is more secure. If I lose the device that has my passkey, I need a way to access my account from another device.

One might say "just use a password/credentials manager", but all these passkey prompts don't emphasize that you should use one that is not tied to hardware, or that you should backup your passkey to a thumb drive, or anything like that.

Same reason I hate TOTP that is tied to a device. If you require me to have proprietary 2FA apps on my phone to login to your account, I don't use 2FA. Looking at you, Steam.