I neither like nor understand this restriction. It makes device failure / loss / theft a much more difficult experience to recover from than it would otherwise be. The device should be throwaway. I specifically keep old phones in case something happens to the new one.
WhatsApp is probably the stupidest example of only being able to be on a single device (but I'm forced to use WhatsApp for one specific purpose, so I already resent it). Signal does the same thing, so maybe it's related to the E2EE that WhatsApp licensed from Signal...
that's not really an artificial limitation but a design choice. They don't store your messages, only deliver them. Once the message is on your device, it's gone from their servers, like old POP3 mail.
As is with all two factor, but don't point that out, or the "but muh security" bros will shout you down.
I would be extremely F'd if my 2FA was able to be lost or stolen due to a single device limitation.