If one of Rivens AD scaling spells did magic damage she would be way stronger

Mixed damage is strong midgame, when people only have one or two defensive items; this is why champs like Corki spike so hard in the midgame (plus Triforce). However, as resistances start to pile up in the late game, the lack of penetration really starts to hurt.

Orianna is an example of a champion that deals pretty much one kind of damage: magic. She's hurt by magic resist, so she builds Void Staff and Sorcerer's shoes to counteract her targets' MR. If we look at Corki, on the other hand, about a third of his lategame damage is magic, but unlike Orianna, he has Berserkers Greaves and a Last Whisper, so that part of his damage gets hit really hard.

Now bringing it back to Riven. Riven isn't great in teamfights; she's a fantastic duelist and splitpusher, but she has to be VERY careful in a 5v5, since melee and squishy do not mix well. With that being so, she could probably get away with dealing magic damage on, say, her ult or something. It would make her level 6 powerspike stronger (since her lane opponent has to build armor to survive her), but as they start to complete more items, she'd begin to fall off; as a split-pusher and duelist, that's alright. Miss Fortune, however, THRIVES in teamfights, and her team is depending on her to deal serious DPS in the lategame, so having a bunch of her damage cut really hurts, even more so than Corki, who at least has high burst and a great escape.