It even says in the tips when you die to her.
LoL is balanced around blowing up your opponent's Nexus, not 1v1 potential.
Some champions are better at 1v1 fights then others. Those tend to be good at 1v2 fights as well.
Some champions are better team-fighters then others. Those tend to be good in 4v4 fights as well.
In most cases fighters are amazing in 1v1 situations and pretty lackluster in 5v5 fights. Riven is no exception there. Her AoE potential can allow her to be ok, but she usually needs to gain a fairly solid lead to really even pull her weight in a team fight.
Riven is at least beatable with a range of champions in a 1v1 too, due in large part to the majority of her kit being pretty telegraphed and avoidable. It's also quite possible to bait her into using her core damage as a gap-closer that deals 0 damage.
There are other fighters in the game where skill plays a much smaller roll when 1v1s are involved, and they beat champions more automatically.
Jax vs. a range of champions is usually pretty one-sided.
So when are people going to realize Riven isn't balanced?
She kind of sucks at the moment, the changes to her core armor penetration items at the beginning of this season really did nothing to help her out.
Fighter and tank itemization as a whole also improved pretty substantially, causing Riven to take 2 steps back where pretty much everyone else in top lane got to take 2 steps forward.
Riven isn't doing well at the moment.