They should have this setting, but I think the main reason they do not is visibility. Everyone has equal visibility in most cases. If you were able to turn weather effects off, there is a chance you would have an advantage over spotting/claiming a mob faster than the player that has weather effects on. The same reason why the game does not have any draw distance effects with sliders. i.e.: seeing nameplates further away, turning off shadows, etc.

BUT I would argue that since this game doesn't really have any competitive elements except in PVP arenas, just have the weather toggle available in all zones EXCEPT Frontlines.