Why do mages have such high base damage on their abilities?

I remember when Riot lowered the AP on nearly every single item, and then said, >We want people to buy a Rabadon's Deathcap if they wish to assassinate people!!

But no one ever does that, do they? You have your champs like with only 200 AP in the first ten minutes, but still manages to slaughter anyone with less than 600 magic resist in half a second with that nasty burst. Those base damages need to be looked at. It's the reason why so many champs can do better with tanky builds - they don't need damage items to do damage.