Runaan's stronger that Wits End ?

Since I started maining Kayle, I settled on a build that I loved:-

I may deviate from that build depending on the situation. When I need more survivability, I may opt out of Void Staff and Wit's End for tank items but for damage builds, I've always included Wit's End.

I discovered early that RageBlade after Nashors was a huge powerspike. But today, I decided to try building Runaan's after Nashors instead and I discovered that it was just as insane a powerspike but much cheaper to reach due to the low price of Runaan's. But what really tipped it over is when I built RageBlade after. Her power was nothing short of pure insanity. She could melt an entire team of squishies and even tankish bruisers in a couple seconds. If I substitute Wit's End for Runaan's in my original build, the difference in her power is noticeable.

I was never a fan of building Runaan's on her because when I compared Runaan's and Wit's End, Wit's End seemed much stronger. Runaan's on-hit passive was only 15 physical damage but Wit's End was a whole 40 magic damage and Wit's End give you an MR shred. The bolts form Runaan's applies on-hits and a measly 25% AD on-hit damage. Yet this item is far more powerful than Wit's End.

My question is why is Runaan's, despite weaker on-hit stats, so much stronger on her ? I can't figure it out.