They're also stupid, bestial and can't reproduce.
And they don't have the whole ocean under surveillance. Their numbers were drastically reduced during the Fracas with Gilgamesh.
Only about a hundred or so left from a good thousand.
“She'd become a governess. It was one of the few jobs a known lady could do. And she'd taken to it well. She'd sworn that if she did indeed ever find herself dancing on rooftops with chimney sweeps she'd beat herself to death with her own umbrella.”
― Hogfather