Nasus Q VS Spellshields

One thing. Nasus Q. The ability with the power to wreck your adc, midlaner, top lane or anyone without a massive amount of health in just a few shots. I've been playing Nocturne and Sivir as two of my strongest champions for a long time. Their mobility is great, their damage is nice, but the best part about them that rounds them out as champions are their ability to block spells. Any ult any stun any slow any anything...except a Nasus Q. So i want to hear it from someone, no one in particular, but someone with the knowledge as to why this is. In these matchups here's the scenario. You're playing sivir farming bot lane, before recalling for that juicy Phantom dancer, right when Nasus teleports in behind you. Option A: continue recalling and pray you make it out. Option B: Run for the hills and hope you make it. And here's the problem, while option A sounds good it's still a pretty sad option, and option B; this requires you to either blow everything to run or predict the Whither and claim you're a magician. Sounds like fun right. So why. the two champs unique to the game with spellshields can they not block this ability? With a fed or farmed nasus it makes it impossible to win trades when you rely on being able to negate costly abilities to survive.

I'm not asking how to counter it or outplay it, I'm simply asking why this ability cannot be blocked, or is made to be this way.

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