This doesn't make sense. As a website author, why would I visibly restrict the security keys to backdoored government keys, when I can simply give the government an invisible backdoor API or direct database access?

This is intentional, given that “self-made” keys have plenty of ways to be insecure, and the average user cannot tell the difference between a well made key and one that is either accidentally or intentionally defective in ways that affect their security.

FIDO2 is designed to maximize security for the majority of users, and the majority of users are using Yubikeys or other hardware-backed tokens provided by big players in their space.