Can't you do that with a prerouting firewall rule?

Genuinely asking, never tried myself but seems plausible.

There are a number of ways you could handle this, but none of them make wireguard seem user friendly for this use case.

If you're using WireGuard for point to point or to access a specific subnet, this isn't an issue.

But a common use case is to use WireGuard like you'd use Mullvad or Nordvpn and tunnel all traffic through it. And if you need exceptions for private address ranges or specific services, you end up having to generate a CIDR list (the WireGuard mobile app can do this for you if you check the "exclude private addresses" checkbox, but no such checkbox exists for wireguard tools on Linux, and it's a hardcoded list anyway), or add routes yourself, or fiddle with firewall rules.