[lifthrasiir]

Because it is required to be monotonic per peer. WireGuard has no intrinsic states, so multiple machines with the same peer key will be seen as a single peer and that is actually a legitimate use of WireGuard. These machines would have to be synchronized to each other (but not necessarily to the external clock), and using a time is a straightforward and reasonable way to ensure this.

[apexalpha]

So could you share a single set of keys on multiple machines and Wireguard will work, as long as all machines use the same ntp server?

[lifthrasiir]

Technically yes, but I don't think (but haven't exactly confirmed) that you can initiate a new session to itself.