Implementation requires different considerations, no? For example, AES-GCM needs to be supplied a big-endian nonce, whilst ChaPoly uses little-endian. As the WireGuard paper notes, ChaPoly (in software) can be better protected against (CPU) side-channels. Besides extended-nonce AEAD being "native" to ChaPoly, BLAKE family of hash functions that WireGuard uses, are also based on the same construction as ChaPoly, making the implementation leaner (something Jason keenly emphasizes as an advantage).