Because they have the same problem: not trading anything for mobility. You have two completely resourceless champions who have very low cooldowns on their dashes. This is why they are so much fun to play, but very few people enjoy playing against them. You can't give a champion four dashes (why does Riven's E even need a dash with it? The world may never know) with cooldowns that are less than 5 seconds and not have them trade anything for it. It is impossible to get away from a Riven, and it is impossible to catch her if you do get an edge up.
If this buff to Riven's ult damage goes through, please for the love of god up her cooldowns to make up for how easy her itemization is right now.
I agree they both have high skillcaps, but it's extremely frustrating to play against champions where it's less about how skilled you are and more about how unskilled they are.