WireGuard is 4k LoC and is very intentional about its choice of using a single, static crypto implementation to drastically reduce its complexity. Technically speaking, it has a lower attack surface for that reason.
That said, I've been on your side of the argument before, and practically speaking you can expose OpenSSH on the public internet with a proper key setup and almost certainly nothing will happen because it's a highly-audited, proven piece of software. Even though it's technically very complex.
But, that still doesn't mean it isn't best practice to avoid exposing it to the public internet. Especially when you can put things in front of it (such as WireGuard) that have a much lower technical complexity, and thus a reduced attack surface.