Because of how it affects success cases, particularly in bot lane.
A lucky crit in the earlygame can completely shift the entire course of laning phase in the favor of a potentially less-skilled player. Sure, over time, it equals out, I don't think anyone is gonna contest that.
The point is that RNG shouldn't decide laning as heavily as it currently can, not necessarily "RNG IS EVIL".
Pretend you're in bot lane, playing ADC, in 2 different scenarios(crit vs no crit). Let's assume you get hit 3 times in the first lets say 10 minutes of laning, equaling the same amount of damage in either scenario.
If you're hit by 3 non-crit autos, you know exactly how much damage you can take, you can still CS and trade, and it doesn't necessarily change the way you approach the lane.
However, if the first of those 3 autos is a crit, you've now taken a large hit to your health, and even if the other 2 autos are non-crits that by themselves do less damage than in the no-crit scenario, you have to play safer. Not only do you not know for sure whether the next autos will be crits or not, you've taken twice the damage for no extra work on the enemy ADC's part, you have to come out on top during trades to reach equal threat levels with the enemy ADC, and depending on the damage level you might feel the need to (depending on when the damage was received) burn heal or your support might use mana/skills they wouldn't otherwise due to the immediate damage spike.
RNG definitely has a place in league, and on paper the numbers are balanced. The issue comes from the negative gameplay experience it causes, particularly in cases where a frontloaded crit in a trade rewards way too much for zero extra work on the "crit-er"s part, even if they sacrifice later damage instances to acquire it.
The "Consistent DPS" versus "Burst Damage" is a legitimate candidate for debate, as both have their benefits. The problem comes from "Burst Damage" being almost universally the superior option with regards to highly trade-focused lanes like Top and Bot where players are usually playing "Consistent DPS" roles, and that shouldn't be the case as much as it is.
Crit should stay, it should just not always be the more powerful option (at least as far as ADCs are concerned). Pure stats say it is not, it's effect on gameplay, laning, and PvP interactions say it is. If nothing else, raw AD should be just as appealing, but IE is such a powerful item right now (as it has been for awhile, since the BT "changes" cough nerf cough) and gives so much AD that rushing IE is almost always superior except in a few fringe cases.
& 
are heavily reliant on critting for their playstyles, as well as one of the most awesome AD items in the game is 
. It should be acceptable for an ADC to decide to opt for a ghostblade, but it shouldn't be better than the %damage amps provided by the reworks.
for melee carries and rework 
to be ADC sustained damage amp.
chimes and their spawns being a cool, reactive form of RNG. Non-interactive RNG is what they don't like. Crit's are very non-interactive because they can't be avoided but they can still change the course of the fight. Good RNG is something that you can't control, but you can react to and make decisions about.
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have been in the game for so long and I disagree with removing them even if they don't work most of the time, they exist and provide unique, fun and different feelings that remain as an option. What disadvantage is there to their existence? Nearly nothing, 
