~~Answer: because she gets ten million shields per second... stun... 4 dashes... etc. (jokes)~~
Real Answer: because she can do extra 150% AD damage from passive ALONE starting at level 1.
_What's her passive?_
Riven's blade gains a Charge for 6 seconds every time she uses an ability (stacks up to 3 times). Riven's basic attacks consume a Charge when available to deal 25% − 50% (based on level) AD bonus physical damage. (Source: LoL Wiki)
Now, it says it stacks up to 3 times, but there is a trick to stack it 6 times at all levels, including level 1.
This means that at level 1, she does 150% AD bonus physical damage.
_Let's do the math:_
Riven's Base Attack Damage: 64 - 115 (based on level)
Let's say you start Doran's Blade (common starting item against melee champions): + 8 AD
(64 + 8) x 1.5 = 108 bonus physical damage at level 1???!
You might think this is low, but dude. Tryndamere, who has the highest base health stat at level 1, has 625.64 HP.
108 is 17% of Trynd's health (of course, not taking armor and starting item into consideration)
How do you do it?_
Cast the second and the third Q as late as possible in order to infinitely keep your passive stacks.
In-depth video guide: https://youtu.be/dz0yx4DeO04
(Although the video includes everything from laning phase, trade timing, mid-late game macro and much more,
accurate description of Runic Blade and explanation of the mentioned trick start playing at 0:27 and last only a bit!)
~~Rito don't nerf her~~