Why is Riven played SO much?

Brainlessly easy to play in addition to having very low risk / high reward gameplay.

Ability to snowball very hard, scales better than 90% of the cast, no weaknesses or trade-offs for her absurd benefits (Manaless but with low cds.... has high mobility in addition to multiple AoE stuns, etc.)

Broken OP is the correct answer. The ONLY reason she isn't perma-banned is the Juggernauts.

1. Because Cho isn't played top often
2. Because Cho isn't played top often

she gets wrecked fk hard cause riot gutted her hp regen shield everything and now ult cd increase wich makes hard to snowball.. a riven dat doesnt snowball tends to feed and afk.. say thx to riot

Fairly easy to play and carry with, and she's absurdly strong overall. She pretty much dic(k)tates how fights will go all the time, and you can't really do much about that (unless it's a 1v5 scenario or she's behind, but even then, she reaches hypercarry state with a longsword at level 2)

Not going to lie she is actually pretty good right now so there is that. Most good champs get picked a lot. Even when she was first changed back in season 4 and she was arguably underpowered she was still really good at snowballing and as a result carrying games especially in ranked.

The other reason is she is fun. There are a lot of people that are just fans of her, either because they like the character her mechanics her learning curve or they are just emulating some streamer. There are a lot of people who will play Riven if she is good or not.

As for why she isn't banned? That is based on who is banning. If the top laner is banning and doesn't ban her its because he is confident in the match up most likely. Unlike, say, the Juggernauts who can kinda just steam roll the lane riven can be outplayed or harassed in some way. Even when riven is strong a better laner can still win the matchup unlike more op/easier champs. Not saying to Riven is "OMG SUPR HARD MUH SKILL" but seeing a bad riven us all her q's and e to just get near someone and then die because she has no follow up is about as common as seeing one stomp.

Because her kit is broken and overloaded.

She's got very attractive theme, backstory, and playstyle - people play her alot even when she's super weak in the meta because they like her. The fact that she's currently sitting at a 60% winrate in super high elo means she's very visible and streams (she even saw some recent play at worlds), and so people start to play her even more despite that she has a sub 50% winrate until almost plat.

She is the definition of an easy to pick up, lifetime to master champion. If you are truly great with her, you really can dictate the pace of a whole game by yourself... or you can fail hard and lose the game almost singehandedly if you aren't as good as you thought and your ego can't handle playing safer.