The flippant answer is because they actually aim at mid-Atlantic!
In effect, the coriolis force doesn't get noticed among all the drift caused by wind, jetstreams etc. Its effects are rather longer-term and the force itself is far smaller, so it works over long term on major air masses.
Somewhere I used to have among my favourites a website that showed all the effects, one by one, on air in the atmosphere, of coriolis, rotation of the earth, heating by the sun, etc. etc. but I can't find it - lost in a HD rebuild, I suspect...
