The rationale is that if there is randomness then random chance can easily determine the outcome of a match (instead of player skill) which is unacceptable for pro play and tournaments. Quoting the articles I linked to above:
In general Randomness is rejected from competitive gaming since it can decide over loss and defeat without reflecting the actual skill level.
In League, a single action can determine the outcome of a lane, or even the game. For this reason, randomness in the game is a tricky topic. While landing a huge critical strike (crit) on your opponent can be satisfying, being the victim of a “lucky” crit can be frustrating, or worse: a random event could cause a skilled player to lose to a less-skilled player, even though the skilled player made a better play. In a casual game, the decision to include randomness might be a no-brainer, if it makes the game more fun. However, the decision to include randomness in Esports has the potential to make or break a pro player’s career or determine the outcome of a tournament. As a result, many game developers take measures to reduce the influence of randomness in their games.
Seems reasonable, but... I dont understand, there just seem to be too many counterexamples! Pokemon is insanely random, yet Pokemon is being played competitively as far as Im aware. More traditionally, Poker and Bridge seem to be ones of the most obvious counterexamples. These games would seem to demonstrate that it is not the case that pro play demands little to no randomness?
If and why does it make sense to reduce randomness in games intended to be played competitively?
For the sake of completeness, I feel I need to include two more opinions on the subject I heard/read.
Firstly: On Discord, someone told me that Poker is not really comparable to LoL as it is less popular (I was astonished to learn that LoL tournaments have tens of million watchers while Poker only has hundreds of thousands) and Poker is a gambling game, which is why randomness works for Poker - randomness in games makes them gambling games which presents a problem on its own.
Secondly: Extra Credits made a video on the subject, where they said randomness can work for competitve games (and IIUC implied that the claim that competitive games must have little to no randomness is incorrect) but tournament format needs to be adjusted - instead of single elimination they should make the tournaments in round robin. But doesn't this introduce new problems, since - correct me if I'm wrong - don't most people, both players and spectators, find round robin excessively boring?