The champs are MECHANICALLY easy. Very easy. It's not their mechanics though that make them high skill. Easy to use kits are very far from easy to use champions. To master a champion, you must have timing, focus, and know your CD's perfectly. Usually getting down your CD's are the easy part, but knowing when to go in on champions like Riven, Kat, Yi, and Tryn is so intensive. You have to know 100% that you can take a specific target for either your reset or put out the needed damage. If you're just going to say "Kat is easy, she's so freelo" and then a noob picks her up, notices her kit is mechanically easy, then tries to dive the backline before her team has done anything, she's going to die and be worthless. If she waits, watches the fight, chooses a target, then PROPERLY goes in on it for the kill and actually gets it, then she's a good Kat. She's able to choose her fights wisely rather than doing what Darius does and walk into a fight, get low, press Q, and not... really do anything mechanically or skillful.
Timing, Prioritizing, and Patience are all part of a champions skill cap. You won't see many Rivens actually doing well if they don't know their champion inside and out unless their opponents are complete idiots and don't know anything about Riven. Same goes for Kat, really. It's why you'll see Rivens beat Olafs and Kats beating LeBlancs. If they have more skill, they'll likely win. It's just how it is. Almost every match up where a Riven or Kat get fed it's because of the enemies not understanding the champ or not dealing with it.
A good example of a team not dealing with Riven was recently in World's! ROC vs. Fnatic. Huni sat top with a free lane against a Rumble. Rumble is a poor champ against a Rumble and Huni was ignored so hard that he had an inner turret at 2 minutes in as RIVEN. ROC had a team to deal with Riven and just ignored her completely despite her being 100 CS above her lane opponent on top of having 2 turrets down, plus getting ganks from a Rek'Sai and a TF on a number of occasions. If ROC had ganked Riven like Fnatic ganked Rumble, Riven would've fell off and Rumble would've dominated in team fights, but instead she was ignored and allowed to get two towers and a number of kills. Huni's Riven wasn't even good either. He played super cocky and would've been destroyed if he didn't have a free lane and two fricken turrets for free.