One simple reason is that the serverside might not want to expose a shell to its clients, and instead just provide network connectivity; you can configure something like that with SSH, but it's a pain, and WireGuard is approximately as simple to set up as SSH, which is the primary reason it's so popular.
Is it a pain? As far as I know, all that's needed is to insert restrict,command="/sbin/nologin",port-forwarding before the user's key in authorized_keys. You can add more security by using a separate user, but individual Unix users for each client are not^W^Wshould not be necessary for security.
