Do you know the reason Janna doesn't get attacked on the boards? Because it's really hard to notice when a Janna is carrying a game.
Everyone knows when an assassin/fighter is carrying a game (
) -- they're 15/0 or some similar scoreline, they do a million damage, they kill people in 0.5 seconds or less, and they're basically unkillable while they solo your team 1v5 by being immune to skillshots. You feel like you never had a chance, like they entered the game 5/0, and like the champion is just unfair.
Everyone knows when an ADC is carrying a game (
) -- they hit you with 2-3 auto attacks and you die. They crit you for over a thousand damage a hit. They can face tank your team because their lifesteal is higher than the damage you deal. That one good teamfight you get, you think you have a chance, but that's only because your Sejuani finally hit an ult on said ADC and you blew her up while she couldn't lifesteal. But then you never get another fight like that and they wreck your team fight after fight.
Everyone knows when a burst mage or an artillery mage is carrying a game (
) -- you get hit by a CC and you know you're instantly dead. Magic resist doesn't seem to matter -- they already have a
before you have two MR items. They're always able to kill 2-4 members of your team. You manage to kill them in a lot of fights, but once they're dead, the fight is 1v4 or 2v4 and you have no chance of winning anyway. Everyone on your team's like "focus <insert mage>" and everyone's like "she's dying but it's not enough, the <insert ADC> is still killing what's left when the mage goes down".
When a Janna is carrying a game, nobody really notices that Janna is the one behind what you're experiencing. You notice a team that always outrotates yours (because Janna's passive gives them movement speed). You notice that you can never quite engage right (because Janna's ult knocks people away). You notice that people are always escaping with a sliver of health instead of dying (because Janna's shield is keeping them alive). You think your support isn't warding (because Janna and her team have vision everywhere). You just assume their team is universally better than yours in every position.
1.4 million people died in car accidents last year (2016). Compare this to the figure for plane crashes (most recent one I could find is 2014, with 992 people). Despite this, most people feel safer driving than flying. This is because any individual plane crash is a huge, noticeable event that makes world news no matter where the crash occurs or what airline it occurred on. Any given car crash, even when fatal, only makes local news. Very few car crashes make national or world news, and when they do, it's almost universally because of harsh winter road conditions (an ice storm that hits a major freeway in USA) or an engineering failure (a large bridge in a major city collapses during rush hour). Though it's never happened before, terrorism would also fall on this list (imagine someone setting off a bomb in a high-traffic tunnel).
Assassins, marksmen, and mages are the plane crashes. Janna is a car accident.