Why Mage Supports Do Most Damage

I hear people say that Zyra and Malz need to be gutted for consistently doing more damage than the mid and top laner with less gold

People seem to forget that Malzahar and Zyra do damage, that's all they do, from the second the game starts to when it ends they're just poking people because unlike mid and top lane, they don't have to worry about last hitting creeps and such, just poke