The thing is she doesn't have exploitable weaknesses by the majority of the game. If you are ranged generally you do not have enough escapes to run away from her. If you are melee you are generally prone to having to skirt the edge of her range unless you have a silence and tank plus damage to back you up.
Darius does well because he has a DoT which helps abuse Riven's weakness. Cho and Garen do well because they can silence her to force trades in their favor. There is only 1 real match up against Riven where skill means everything and that is Irelia. And both snowball on their first kill. Gnar can sometimes beat Riven but his hop on a 20s CD makes it really hard to keep away all that long because she just has to try again.
The point is the tactics to Riven are to somehow deny her CS or kill her. But she meanwhile is zoning you from the farm and choking you out unless you counter her. And unless you have a hyper gank jungler like Lee or Amumu it is near impossible to catch her out.