What ELO are you in?
Champs with incredibly strong early game damage and high snowball potential, like
, are popular in low ELO because those players don't understand how these champs work. Fiora and Riven are extremely difficult to outdamage or outplay pre-level 6, and when fed they are capable of snowballing extremely quickly and with a couple more kills will become a roaming menace that can singlehandedly destroy your entire team in the blink of an eye.
The thing about riven and fiora is that because they have very little utility and are reliant on items for mid-game damage, they fall off extremely hard if not fed. All it takes to shutdown riven and fiora is to not give them any kills early. Then once you have a point in your ult, let them push to your turret, then get your jungler to come gank you and let loose with all your abilities to kill them before they can escape, You will get a kill, jungler an assist, and riven/fiora will be useless for the rest of the game because they are behind in kills.
Of course a lot of low ELO players are stubborn in the belief that they can outdamage a riven/fiora without an ult, and build offensively and try to snag a first blood on them, almost always ending up utterly failing in doing so. They don't understand that rushing armor will completely ruin any fiora/riven , and don't understand that being 0/0 hurts fiora/riven much, much more than being 0/0 hurts a more utility based champ.
Playing riven in low elo usually results in you getting fed early, leading to a mega-snowball and an easy win.
It's the same problem with LB, honestly i'm tired of losing my lane when a fed LeBlanc comes for a visit because our midlaner is too stubborn to come to terms with the fact that LeBlanc is probably going to far more damage than they do early game, and that denying LeBlanc an early kill causes her to fall off massively mid and late game.