I have like 2 or 3 physical keys. I have accounts on hundreds of websites. Sure, you can use the same key on every website but you better not lose it. So now yo

I have like 2 or 3 physical keys. I have accounts on hundreds of websites. Sure, you can use the same key on every website but you better not lose it. So now you need to register multiple keys on every website. Also if you lose one you need to go back to every single site and add your replacement. (I hope you remember every site you have signed into.) So "not copyable" is actually a huge downside for convenience. Such a downside that even though I have a collection of U2F keys I only use them in a handful of accounts. The maintenance cost is just far too high. To resolve this you would probably need something like cross-signing. So I can say "I know that you only trust key A, but I lost it a few years ago. However I have an attestation from key A saying that key B is mine as well. Here is a signature from key B". However this is effectively equivalent to copying keys. So it basically defeats that point.