No one expects the Spanish Inquisition!
everyone knows that splash is a OHKO move...no one has seen it activate yet...
...look, if they are stupid enough to expect a Kyogre with Thunder, then they may as well start teaching their Pikachu's earthquake*...no one logically should learn a move that is super effective against themselves...
*this is not a reference to the anime, where Pikachu knows Earthquake, Fire Blast, Surf, Draco Meteor, Fly, Dash Attack, Thunder, and Thundershock, in that order